**1) 1**

1 may be the loneliest number but it’s also a very important number for divisibility! Every integer is divisible by 1, and the result of any integer x divided by 1 is just x (when you divide an integer by 1, it stays the same). On the GMAT, the fact that every integer is divisible by 1 can be quite important. For example, a question might ask (as at least one official problem does): Does integer x have any factors y such that 1 < y < x?

Because every integer is divisible by itself and 1, that question is really just asking, “Is x prime or not?” because if there is a factor y that’s between 1 and x, x is not prime, and there is not such a factor, then x is prime. That “> 1″ caveat in the problem may seem obtuse, but when you understand divisibility by 1, you can see that the abstract question stem is really just asking you about prime vs. not-prime as a number property. The concept that all integers are divisible by 1 may seem basic, but keeping it top of mind on the GMAT can be extremely helpful.

**2) 2**

It takes 2 to make a thing go right…in relationships and on the GMAT! A number is divisible by 2 if that number is even (and a number is even if it’s divisible by 2). That means that if an integer ends in 0, 2, 4, 6, or 8, you know that it’s divisible by 2. And here’s a somewhat-surprising fact: the number 0 is even! 0 is divisible by 2 with no remainder (0/2 = 0), so although 0 is neither positive nor negative it fits the definition of even and should therefore be something you keep in mind because 0 is such a unique number.

The GMAT frequently tests even/odd number properties, so you should make a point to get to know them. Because any even number is divisible by 2 (which also means that it can be written as 2 times an integer), an even number multiplied by any integer will keep 2 as a factor and remain even. So even x even = even and even x odd = even.

**3) 3**

It’s been said that good things come in 3s, and divisibility rules are no exception! The divisibility rule for 3 works much like a magic trick and is one that you should make sure is top of mind on test day to save you time and help you unravel tricky numbers. The rule: if you sum the digits of an integer and that sum is divisible by 3, then that integer is divisible by 3. For example, consider the integer 219. 2 + 1 + 9 = 12 which is divisible by 3, so you know that 219 is divisible by 3 (it’s 3 x 73).

This rule can help you in many ways. If you were asked to determine whether a number is prime, for example, and you can see that the sum of the digits is a multiple of 3, you know immediately that it’s not prime without having to do the long division to prove it. Or if you had a messy fraction to reduce and noticed that both the numerator and denominator are divisible by 3, you can use that rule to begin reducing the fraction quickly. The GMAT tests factors, multiples, and divisibility quite a bit, so this is a critical rule to have at your disposal to quickly assess divisibility. And since 1 out of every 3 integers is divisible by 3, this rule will help you out frequently!

**4) 4**

Presidential Election and Summer Olympics enthusiasts, be four-warned! You already know the divisibility rule for 4: take the last two digits of an integer and treat them as a two-digit number, and if that’s divisible by 4 so is the whole number. So for 2016 – next year and that of the next presidential election and Brazil Olympics – the last two-digit number, 16, is divisible by 4, so you know that 2016 is also divisible by 4.

If you fail to see immediately that a number is divisible by 4 given that rule, fear not! Being divisible by 4 just means that a number is divisible by 2 twice. So if you didn’t immediately see that you could factor a 4 out of 2016 (it’s 504 x 4), you could divide by 2 (2 x 1008) and then divide by 2 again (2 x 2 x 504) and end up in the same place without too much more work.

**5) 5**

Who needs only 5 fingers to divide by 5? All of us – divisibility by 5 is so easy you should be able to do it with one hand tied behind your back! If an integer ends in 5 or 0 you know that it’s divisible by 5 (and we’ll talk more about what extra fact 0 tells you in just a bit…).

**6) 6**

Your favorite character from the hit 1990’s NBC sitcom “Blossom” is also an easy-to-use divisibility rule! Since 6 is just the product of 2 and 3 (2 x 3 = 6), if a number meets the divisibility rules for both 2 (it’s even) and 3 (the sum of the digits is divisible by 3) it’s divisible by 6. So if you need to reduce a number like 324, you might want to start by dividing by 6, instead of by 2 or 3, so that you can factor it in fewer steps.

**7) 7**

Ah, magnificent 7. While there is a “trick” for divisibility by 7, 7 occurs much less frequently in divisibility-based problems (as do other primes like 11, 13, 17, etc.), so 7 is a good place to begin to think about a strategy that works for all numbers, rather than memorizing limited-use tricks for each number. To test whether a large number, such as 231, is divisible by 7, find an obvious multiple of 7 nearby and then add or subtract multiples of 7 to see whether doing so will land on that number. For 231, you should recognize that a nearby multiple of 7 is 210 (you know 21 is 7 x 3, so putting a 0 on the end of it just means that 210 is 7 x 30). Then as you add 7s to get there, you go to 217, then to 224, then to 231. So in your head you can see that 231 is 3 more 7s than 7 x 30 (which you know is 210), so 231 = 7 x 33.

**8) 8**

8 is enough! As you saw above with 4s and 6s, when you start working with non-prime factors it’s often easier to just divide out the smaller prime factors one at a time than to try to determine divisibility by a larger composite number in one fell swoop. Since 8 = 2 x 2 x 2, you’ll likely find more success testing for divisibility by 8 by just dividing by 2, then dividing by 2 again, then dividing by 2 a third time. So for a number like 312, rather than working through long division to divide by 8, just divide it in half (156) then in half again (78) then in half again (39), and you’ll know that 312 = 39 x 8.

**9) 9**

While “nein” may be German for “no,” you should be saying “yes” to divisibility by nine! 9 shares a big similarity with 3 in that a sum-of-the-digits rule applies here too. If you sum the digits of an integer and that sum is a multiple of 9, the integer is also divisible by 9. So, for example, with the number 729, because 7 + 2 + 9 = 18, you know that 729 is divisible by 9 (it’s 81 x 9, which actually is 9 to the 3rd power).

**10) 10**

We’ve saved the best for last! If a number ends in 0, it’s divisible by 10, giving you a great opportunity to make the math easy. For example, a number like 210 (which you saw above) lets you pull the 0 aside and say that it’s 21 x 10, which means that it’s 3 x 7 x 10.

Working with 10s makes mental (or pencil-and-paper) math quick and convenient, so you should seek out opportunities to use such numbers in your calculations. For example, look at 693: If you add 7, you get to a number that ends in two 0s (so it’s 7 x 10 x 10), meaning that you know that 693 is divisible by 7 (it’s 7 away from an easy multiple of 7) *and* that it’s 7 x 99 because it’s one less 7 than 7 x 100. Because the GMAT rewards quick mental math, it’s a good idea to quickly check for, “If I have to add x to get to the nearest 0, then does that give me a multiple of x?” (297 is 3 away from 300, so you know that 297 = 99 x 3). And since 10 = 2 x 5, it’s also helpful sometimes to double a number that ends in 5 (try 215, which times 2 = 430) to see how many 10s you have (43). That tells you that 215 = 43 x 5 because 215 x 2 = 43 x (2 x 5). Working with 10s can make mental math extremely quick – we’d rate numbers that end in 0 a perfect 10!

*Getting ready to take the GMAT? We have free online GMAT seminars running all the time. And, be sure to find us on Facebook and Google+, and follow us on Twitter!*

*By Brian Galvin*

Small numbers on your noteboard.

Have you found yourself on a homework problem or practice test asking yourself “can I just multiply these?”? Have you forgotten a rule and wondered whether you could trust your memory? Small numbers can be hugely valuable in these situations. Consider this example:

For integers x and y, 2^x + 2^y = 2^30. What is the sum x + y?

(A) 30

(B) 40

(C) 50

(D) 58

(E) 64

Every fiber of your being might be saying “can I just add x + y and set that equal to 30?” but you’re probably at least unsure whether you can do that. How do you definitively tell whether you can do that? Test the relationship with small numbers. 2^30 is far too big a number to fathom, but 2^6 is much more convenient. That’s 64, and if you wanted to set the problem up that way:

2^x + 2^y = 2^6

You can see that using combinations of x and y that add to 6 won’t work. 2^3 + 2^3 is 8 + 8 = 16 (so not 64). 2^5 + 2^1 is 32 + 2 = 34, which doesn’t work either. 2^4 + 2^2 is 16 + 4 = 20, so that doesn’t work. And 2^0 + 2^6 is 1 + 64 = 65, which is closer but still doesn’t work. Using small numbers you can prove that that step you’re wondering about – just adding the exponents – isn’t valid math, so you can avoid doing it. **Small numbers help you test a rule that you aren’t sure about!**

That’s one of two major themes with testing small numbers. 1) Small numbers are great for testing rules. And 2) Small numbers are great for finding patterns that you can apply to bigger numbers. To demonstrate that second point about small numbers, let’s return to the problem. 2^30 again is a number that’s too big to deal with or “play with,” but 2^6 is substantially more manageable. If you want to get to:

2^x + 2^y = 2^6, think about the powers of 2 that are less than 2^6:

2^1 = 2

2^2 = 4

2^3 = 8

2^4 = 16

2^5 = 32

2^6 = 64

Here you can just choose numbers from the list. The only two that you can use to sum to 64 are 32 and 32. So the pairing that works here is 2^5 + 2^5 = 2^6. Try that again with another number (what about getting 2^x + 2^y = 2^5? Add 16 + 16 = 32, so 2^4 + 2^4), and you should start to see the pattern. To get 2^x + 2^y to equal 2^z, x and y should each be one integer less than z. So to get back to the bigger numbers in the problem, you should now see that to get 2^x + 2^y to equal 2^30, you need 2^29 + 2^29 = 2^30. So x + y = 29 + 29 = 58, answer choice D.

The lesson? When problems deal with unfathomably large numbers, it can often be quite helpful to test the relationship using small numbers. That way you can see how the pieces of the puzzle relate to each other, and then apply that knowledge to the larger numbers in the problem. The GMAT thrives on abstraction, presenting you with lots of variables and large numbers (often exponents or factorials), but you can counter that abstraction by using small numbers to make relationships and concepts concrete.

So make sure that small numbers are a part of your toolkit. When you’re unsure about a rule, test it with small numbers; if small numbers don’t spit out the result you’re looking for, then that rule isn’t true. But if multiple sets of small numbers do produce the desired result, you can proceed confidently with that rule. And when you’re presented with a relationship between massive numbers and variables, test that relationship using small numbers so that you can teach yourself more concretely what the concept looks like.

The best way to make sure that your GMAT score report contains big numbers? Use lots of small numbers in your scratchwork.

*Getting ready to take the GMAT? We have free online GMAT seminars running all the time. And, be sure to find us on Facebook and Google+, and follow us on Twitter!*

*By Brian Galvin*

But on the radio this morning – just like on your GMAT exam – there was no Eazy-E. Logistically that’s because – as the Bone Thugs & Harmony classic “Tha Crossroads” commemorated – Eazy passed away about 20 years ago. But in GMAT strategy form, Eazy’s absence speaks even louder than his vocals on his NWA and solo tracks. “No Eazy-E” should be a mantra at the top of your mind when you take the GMAT, because on Data Sufficiency questions, choice E – the statements together are not sufficient to solve the problem – will not be given to you all that easily (Data Sufficiency “E” answers, like the Boyz in the Hood, are always hard).

Think about what answer choice E really means: it means “this problem cannot be solved.” But all too often, examinees choose the “Eazy-E,” meaning they pick E when “I can’t do it.” And there’s a big chasm. “It cannot be solved” means you’ve exhausted the options and you’re maybe one piece of information (“I just can’t get rid of that variable”) or one exception to the rule (“but if x is a fraction between 0 and 1…”) that stands as an obstacle to directly answering the question. Very rarely on problems that are above average difficulty is the lack of sufficiency a wide gap, meaning that if E seems easy, you’re probably missing an application of the given information that would make one or both of the statements sufficient. The GMAT just doesn’t have an incentive to reward you for shrugging your shoulders and saying “I can’t do it;” it does, however, have an incentive to reward those people who can conclusively prove that seemingly insufficient information can actually be packaged to solve the problem (what looks like E is actually A, B, C, or D) and those people who can look at seemingly sufficient information and prove why it’s not actually quite enough to solve it (the “clever” E).

So as a general rule, you should always be skeptical of Eazy-E.

Consider this example:

A shelf contains only Eazy-E solo albums and NWA group albums, either on CD or on cassette tape. How many albums are on the shelf?

(1) 2/3 of the albums are on CD and 1/4 of the albums are Eazy-E solo albums.

(2) Fewer than 30 albums are NWA group albums and more than 10 albums are on cassette tape.

(A) Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient to answer the question asked

(B) Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient to answer the question asked

(C) Both statements (1) and (2) TOGETHER are sufficient to answer the question asked; but NEITHER statement ALONE is sufficient

(D) EACH statement ALONE is sufficient to answer the question asked

(E) Statements (1) and (2) TOGETHER are NOT sufficient to answer the question asked, and additional data specific to the problem are needed

Statistically on this problem (the live Veritas Prep practice test version uses hardcover and paperback books of fiction or nonfiction, but hey it’s Straight Outta Compton day so let’s get thematic!), almost 60% of all test-takers take the Eazy-E here, presuming that the wide ranges in statement 2 and the ratios in statement 1 won’t get the job done. But a more astute examinee is skeptical of Eazy-E and knows to put in work! Statement 1 actually tells you more than meets the eye, as it also tells you that:

- 1/3 of the albums are on cassette tape
- 3/4 of the albums are NWA albums
- The total number of albums must be a multiple of 12, because that number needs to be divisible by 3 and by 4 in order to create the fractions in statement 1

So when you then add statement 2, you know that since there are more than 10 albums total (because at least 11 are cassette alone) so the total number could be 12, 24, 36, 48, etc. And then when you apply the ratios you realize that since the number of NWA albums is less than 30 and that number is 3/4 of the total, the total must be less than 40. So only 12, 24, and 36 are possible. And since the number of cassettes has to be greater than 10, and equate to 1/3 of the total, the total must then be more than 30. So the only plausible number is 36, and the answer is, indeed, C.

Strategically, being wary of Eazy-E tells you where to invest your time. If E seems too easy, that means that you should spend the extra 30-45 seconds seeing if you can get started using the statements in a different way. So learn from hip hop’s first billionaire, Dr. Dre, who split with Eazy long ago and has since seen his business success soar. Avoid Eazy-E and as you drive home from the GMAT test center you can bask in the glow of those famous Ice Cube lyrics, “I gotta say, today was a good day.”

Getting ready to take the GMAT? We have free online GMAT seminars running all the time. And, be sure tofind us on Facebook and Google+, and follow us on Twitter!

*By Brian Galvin*

Because if his interaction with a young, milk-drinking fan is any indication, Harbaugh understands one of the key secrets to success on the GMAT Quant section:

Harbaugh, who told HBO Real Sports this summer about his childhood plan to grow to over 6 feet tall – great height for a quarterback – by drinking as much milk as humanly possible – is a fan of all kinds of milk: chocolate, 2%… But as he tells the young man, the ideal situation for growing into a Michigan quarterback is drinking whole milk, just as the ideal way to attend the Ross School of Business a few blocks from Harbaugh’s State Street office is to use whole *numbers* on the GMAT.

The main reason? You can’t use a calculator on the GMAT, so while your Excel-and-calculator-trained mind wants to say calculate “75% of 64″ as 0.75 * 64, the key is to think in terms of whole numbers whenever possible. In this case, that means calling “75%” 3/4, because that allows you to do all of your calculations with the whole numbers 3 and 4, and not have to set up decimal math with 0.75. Since 64/4 is cleanly 16 – a whole number – you can calculate 75% by dividing by 4 first, then multiplying by 3: 64/4 is 16, then 16 * 3 = 48, and you have your answer without ever having to deal with messier decimal calculations.

This concept manifests itself in all kinds of problems for which your mind would typically want to think in terms of decimal math. For example:

With percentages, 25% and 75% can be seen as 1/4 and 3/4, respectively. Want to take 20%? Just divide by 5, because 0.2 = 1/5.

If you’re told that the result of a division operation is X.4, keep in mind that the decimal .4 can be expressed as 2/5, meaning that the divisor has to be a multiple of 5 and the remainder has to be even.

If at some point in a calculation it looks like you need to divide, say, 10 by 4 or 15 by 8 or any other type of operation that would result in a decimal, wait! Leaving division problems as improper fractions just means that you’re keeping two whole numbers handy, and knowing the GMAT at some point you’ll end up having to multiply or divide by a number that lets you avoid the decimal math altogether.

So learn from Jim Harbaugh and his obsession with whole milk. Whole milk may be the reasons that his football dreams came true; whole numbers could be a major reason that your business school dreams come true, too.

Getting ready to take the GMAT? We have free online GMAT seminars running all the time. And, be sure tofind us on Facebook and Google+, and follow us on Twitter!

*By Brian Galvin*

But we also support everyone’s desire to leave no stone unturned in pursuit of a high GMAT score and everyone’s intellectual curiosity with regard to computer-adaptive testing. So with the full disclosure that these items won’t help you game the system and that your best move is to turn that intellectual curiosity toward mastering GMAT concepts and strategies, here are four major reasons that your response pattern — did you miss more questions early in the test vs. late in the test; did you miss consecutive questions or more sporadic questions, etc. — won’t help you predict your score:

**1) The all-important A-parameter.**

Item Response Theory incorporates three metrics for each “item” (or “question” or “problem”): the B parameter is the closest measurement to pure “difficulty”. The C parameter is essentially a measure of likelihood that a correct answer can be guessed. And the A parameter tells the scoring system how much to weight that item. Yes, some problems “count” more than others do (and not because of position on the test).

Why is that? Think of your own life; if you were going to, say, buy a condo in your city, you’d probably ask several people for their opinion on things like the real estate market in that area, mortgage rates, the additional costs of home ownership, the potential for renting it if you were to move, etc. And you’d value each opinion differently. Your very risk-loving friend may not have the opinion to value highest on “Will I be able to sell this at a profit if I get transferred to a new city?” (his answer is “The market always goes up!”) whereas his opinion on the neighborhood itself might be very valuable (“Don’t underestimate how nice it will be to live within a block of the blue line train”). Well, GMAT questions are similar: Some are extremely predictive (e.g. 90% of those scoring over 700 get it right, and only 10% of those scoring 690 or worse do) and others are only somewhat predictive (60% of those 700+ get this right, but only 45% of those below 700 do; here getting it right whispers “above 700″ whereas before it screams it).

So while you may want to look at your practice test and try to determine where it’s better to position your “misses,” you’ll never know the A-values of any of the questions, so you just can’t tell which problems impacted your score the most.

**2) Content balancing.**

OK, you might then say, the test should theoretically always be trying to serve the highest value questions, so shouldn’t the larger A-parameters come out first? Not necessarily. The GMAT values balanced content to a very high degree: It’s not fair if you see a dozen geometry problems and your friend only sees two, or if you see the less time-consuming Data Sufficiency questions early in the test while someone else budgets their early time on problem solving and gets a break when the last ten are all shorter problems. So the test forces certain content to be delivered at certain times, regardless of whether the A-parameter for those problems is high or low. By the end of the test you’ll have seen various content areas and A-parameters… You just won’t know where the highest value questions took place.

**3) Experimental items.**

In order to know what those A, B, and C parameters are, the GMAT has to test its questions on a variety of users. So on each section, several problems just won’t count — they’re only there for research. And this can be true of practice tests, too (the Veritas Prep tests, for example, do contain experimental questions). So although your analysis of your response pattern may say that you missed three in a row on this test and gotten eight right in a row on the other, in reality those streaks could be a lot shorter if one or more of those questions didn’t count. And, again, you just won’t know whether a problem counted or not, so you can’t fully read into your response pattern to determine how the test should have been scored.

**4) Item delivery vs. Score calculation.**

One common prediction people make about GMAT scoring is that missing multiple problems in a row hurts your score substantially more than missing problems scattered throughout the test. The thinking goes that after one question wrong the system has to reconsider how smart it thought you were; then after two it knows for sure that you’re not as smart as advertised; and by the third it’s in just asking “How bad is he?” In reality, however, as you’ve read above, the “get it right –> harder question; get it wrong –> easier question” delivery system is a bit more nuanced and inclusive of experimentals and content balancing than people think. So it doesn’t work quite like the conventional wisdom suggests.

What’s more, even when the test delivers you an easier question and then an even easier question, it’s not directly calculating your score question by question. It’s estimating your score question-by-question in order to serve you the most meaningful questions it can, but it calculates your score by running its algorithm across all questions you’ve seen. So while missing three questions in a row might lower the current estimate of your ability and mean that you’ll get served a slightly easier question next, you can also recover over the next handful of questions. And then when the system runs your score factoring in the A, B, and C parameters of all of your responses to “live” (not experimental) questions, it doesn’t factor in the order in which those questions were presented — it only cares about the statistics. So while it’s certainly a good idea to get off to a good start in the first handful of problems and to avoid streaks of several consecutive misses, the rationale for that is more that avoiding early or prolonged droughts just raises your degree of difficulty. If you get 5 in a row wrong, you need to get several in a row right to even that out, and you can’t afford the kinds of mental errors that tend to be common and natural on a high-stakes exam. If you do manage to get the next several right, however, you can certainly overcome that dry spell.

In summary, it’s only natural to look at your practice tests and try to determine how the score was calculated and how you can use that system to your advantage. In reality, however, there are several unseen factors that affect your score that you just won’t ever see or know, so the best use of that curiosity and energy is learning from your mistakes so that the computer — however it’s programmed — has no choice but to give you the score that you want.

Getting ready to take the GMAT? We have free online GMAT seminars running all the time. And, be sure to find us on Facebook and Google+, and follow us on Twitter!

*By Brian Galvin*

*On challenging Strengthen and Assumption questions, the correct answer often tells you that a potential flaw with the argument is not true. *

Everything that’s not true in that answer choice, then, makes the conclusion substantially more valid.

Consider this argument, for example:

Kanye received the most votes for the “Best Hip Hop Artist” award at the upcoming MTV Video Music Awards, so Kanye will be awarded the trophy for Best Hip Hop Artist.

If this were the prompt for a question that asked “Which of the following is an assumption required by the argument above?” a correct answer might read:

A) The Video Music Award for “Best Hip Hop Artist” is not decided by a method other than voting.

And the function of that answer choice is to tell you what’s not true (“everything I’m not”), removing a flaw that allows the conclusion to be much more logically sound (“…made me everything I am.”) These answer choices can be challenging in context, largely because:

1) Answer choices that remove a flaw can be difficult to anticipate, because those flaws are usually subtle.

2) Answer choices that remove a flaw tend to include a good amount of negation, making them a bit more convoluted.

In order to counteract these difficulties, it can be helpful to use “Everything I’m not made me everything I am” to your advantage. If what’s NOT true is essential to the conclusion’s truth, then if you consider the opposite – what if it WERE true – you can turn that question into a Weaken question. For example, if you took the opposite of the choice above, it would read:

The VMA for “Best Hip Hop Artist” is decided by a method other than voting.

If that were true, the conclusion is then wholly unsupported. So what if Kanye got the most votes, if votes aren’t how the award is determined? At that point the argument has no leg to stand on, so since the opposite of the answer directly weakens the argument, then you know that the answer itself strengthens it. And since we’re typically all much more effective as critics than we are as defenders, taking the opposite helps you to do what you’re best at. So consider the full-length problem:

**Editor of an automobile magazine:** The materials used to make older model cars (those built before 1980) are clearly superior to those used to make late model cars (those built since 1980). For instance, all the 1960’s and 1970’s cars that I routinely inspect are in surprisingly good condition: they run well, all components work perfectly, and they have very little rust, even though many are over 50 years old. However, almost all of the late model cars I inspect that are over 10 years old run poorly, have lots of rust, and are barely fit to be on the road.

Which of the following is an assumption required by the argument above?

A) The quality of materials used in older model cars is not superior to those used to make other types of vehicles produced in the same time period.

B) Cars built before 1980 are not used for shorter trips than cars built since then.

C) Manufacturing techniques used in modern automobile plants are not superior to those used in plants before 1980.

D) Well-maintained and seldom-used older model vehicles are not the only ones still on the road.

E) Owners of older model vehicles take particularly good care of those vehicles.

First notice that several of the answer choices (A, B, C, and D) include “is not” or “are not” and that the question stem asks for an assumption. These are clues that you’re dealing with a “removes the flaw” kind of problem, in which what is not true (in the answer choices) is essential to making the conclusion of the argument true. Because of that, it’s a good idea to take the opposites of those answer choices so that instead of removing the flaw in a Strengthen/Assumption question, you’re introducing the flaw and making it a Weaken. When you do that, you should see that choice D becomes:

D) Well-maintained and seldom-used older model vehicles ARE the only ones still on the road.

If that’s the case, the conclusion – “the materials used to make older cars are clearly superior to those used in newer ones” – is proven to be flawed. All the junkers are now off the road, so the evidence no longer holds up; you’re only seeing well-working old cars because they’re the most cared-for, not because they were better made in the first place.

And in a larger context, look at what D does ‘reading forward': if it’s not only well-maintained and seldom-driven older cars on the road, then you have a better comparison point. So what’s not true here makes the argument everything it is. But dealing in “what’s not true” can be a challenge, so remember that you can take the opposite of each answer choice and make this “Everything I’m Not” assumption question into a much-clearer “Everything I Am” Weaken question.

Are you studying for the GMAT? We have free online GMAT seminars running all the time. And, be sure to find us on Facebook and Google+, and follow us on Twitter!

*By Brian Galvin*

You’re in luck. Function problems on the GMAT are essentially Shake and Bake recipes. Consider the example:

f(x) = x^2 – 80

If that gets your heart rate and stress level up, you’re not alone. Function notation just looks challenging. But it’s essentially Shake and Bake if you dissect what a function looks like.

The f(x) portion tells you about your input. f(x) = ________ means that, for the rest of that problem, whatever you see in parentheses is your “input” (just like the chicken in your Shake and Bake).

What comes after the equals sign is the recipe. It tells you what to do to your input to get the result. Here f(x) = x^2 – 80 is telling you that whatever your input, you square it and then subtract 80 and that’s your output. …And that’s it.

So if they ask you for:

What is f(9)? (your input is 9), then f(9) = 9^2 – 80, so you square 9 to get 81, then you subtract 80 and you have your answer: 1.

what is f(-4)? (your input is -4), then f(4) = 4^2 – 80, which is 16 – 80 = -64.

What is f(y^2)? (your input is y^2), then f(y^2) = (y^2)^2 – 80, which is y^4 – 80.

What is f(Rick Astley), then your input is Rick Astley and f(Rick Astley) = (Rick Astley)^2 – 80.

It really doesn’t matter what your input is. Whatever the test puts in the parentheses, you just use that as your input and do whatever the recipe says to do with it. So for example:

f(x) = x^2 – x. For which of the following values of a is f(a) > f(8)?

I. a = -8

II. a = -9

III a = 9

A. I only

B. II only

C. III only

D. I and II only

E. I, II, and III

While this may look fairly abstract, just consider the inputs they’ve given you. For f(8), just put 8 wherever that x goes in the “recipe” f(x) = x^2 – x:

f(8) = 8^2 – 8 = 64 – 8 = 56

And then do the same for the three other possible values:

I. f(-8) means put -8 wherever you see the x: (-8)^2 – (-8) = 64 + 8 = 72, so f(-8) > f(8).

II. f(-9) means put -9 wherever you see the x: (-9)^2 – (-9) = 81 + 9 = 90, so f(-9) > f(8)

III. f(9) means put a 9 wherever you see the x: 9^2 – 9 = 81 – 9 = 72, so f(9) > f(8), and the answer is E.

Ultimately with functions, the notation (like the Shake and Bake ingredients) is messy, but with practice the recipes become easy to follow. What goes in the parentheses is your input, and what comes after the equals sign is your recipe. Follow the steps, and you’ll end up with a delicious GMAT quant score.

Are you studying for the GMAT? We have free online GMAT seminars running all the time. And, be sure to find us on Facebook and Google+, and follow us on Twitter!

*By Brian Galvin*

**Checking Whether A Number Is Prime**

Consider a number like 133. Is that number prime? The first three prime numbers (2, 3, and 5) are easy to check to see whether they are factors (if any are, then the number is clearly not prime):

2 – this number is not even, so it’s not divisible by 2.

3 – the sum of the digits (an important rule for divisibility by three!) is 7, which is not a multiple of 3 so this number is not divisible by 3.

5 – the number doesn’t end in 5 or 0, so it’s not divisible by 5

But now things get a bit trickier. There *are*, in fact, (separate) divisibility “tricks” for 7 and 11. But they’re relatively inefficient compared with a universal strategy. Find a nearby multiple of the target number, then add or subtract multiples of that target number. If we want to test 133 to see whether it’s divisible by 7, we can quickly go to 140 (you know this is a multiple of 7) and then subtract by 7. That’s 133, so you know that 133 is a multiple of 7. (Doing the same for 11, you know that 121 is a multiple of 11 – it’s 11-squared – so add 11 more and you’re at 132, so 133 is not a multiple of 11)

This is even more helpful when, for example, a question asks something like “how many prime numbers exist between 202 and 218?”. By finding nearby multiples of 7 and 11:

210 is a multiple of 7, so 203 and 217 are also multiples of 7

220 is a multiple of 11, so 209 is a multiple of 11

You can very quickly eliminate numbers in that range that are not prime. And since none of the even numbers are prime and neither are 205 and 215 (the ends-in-5 rule), you’re left only having to check 207 (which has digits that sum to a multiple of 3, so that’s not prime), 211 (more on that in a second), and 213 (which has digits that sum to 6, so it’s out).

So that leaves the process of testing 211 to see if it has any other prime factors than 2, 3, 5, 7, and 11. Which may seem like a pretty tall order. But here’s an important concept to keep in mind: you only have to test prime factors up to the square root of the number in question. So for 211, that means that because you should know that 15 is the square root of 225, you only have to test primes up to 15.

Why is that? Remember that factors come in pairs. For 217, for example, you know it’s divisible by 7, but 7 has to have a pair to multiply it by to get to 217. That number is 31 (31 * 7 = 217). So whatever factor you find for a number, it has to multiply with another number to get there.

Well, consider again the number 211. Since 15 * 15 is already bigger than 211, you should see that for any number bigger than 15 to be a factor of 211, it has to pair with a number smaller than 15. And as you consider the primes up to 15, you’re already checking all those smaller possibilities. That allows you to quickly test 211 for divisibility by 13 and then you’re done. And since 211 is not divisible by 13 (you could do the long division or you could test 260 – a relatively clear multiple of 13 – and subtract 13s until you get to or past 211: 247, 234, 221, and 208, so 211 is not a multiple of 13. Therefore 211 is prime.

Are you studying for the GMAT? We have free online GMAT seminars running all the time. And, be sure to find us on Facebook and Google+, and follow us on Twitter!

*By Brian Galvin*

**You’re better than that.**

Too often on Data Sufficiency problems, people are impressed by giving themselves a 33% or even 50% chance of success. Keep in mind that guessing one of three remaining choices means you’re probably going to get that problem wrong! And of more strategic importance is this: if one statement is dead obvious, you haven’t just raised your probability of guessing correctly – you *should* have just learned what the question is all about!

**If a Data Sufficiency statement is clearly insufficient, it’s arguably the most important part of the problem.**

Consider this example:

What is the value of integer z?

(1) z represents the remainder when positive integer x is divided by positive integer (x – 1)

(2) x is not a prime number

Many examinees will be thrilled here to see that statement 2 is nowhere near sufficient, therefore meaning that the answer must be A, C, or E – a 33% chance of success! But a more astute test-taker will look closer at statement 2 and think “this problem is likely going to come down to whether it matters that x is prime or not” and then use that information to hold Statement 2 up to that light.

For statement 1, many will test x = 5 and (x – 1) = 4 or x = 10 and (x – 1) = 9 and other combinations of that ilk, and see that the result is usually (always?) 1 remainder 1. But a more astute test-taker will see that word “prime” and ask themselves why a prime would matter. And in listing a few interesting primes, they’ll undoubtedly check 2 and realize that if x = 2 and (x – 1) = 1, the result is 1 with a remainder of 0 – a different answer than the 1, remainder 1 that usually results from testing values for x. So in this case statement 2 DOES matter, and the answer has to be C.

Try this other example:

What is the value of x?

(1) x(x + 1) = 2450

(2) x is odd

Again, someone can easily skip ahead to statement 2 and be thrilled that they’re down to three options, but it pays to take that statement and file it as a consideration for later: when I get my answer for statement 1, is there a reason that even vs. odd would matter? If I get an odd solution, is there a possible even one?

Factoring 2450 leaves you with consecutive integers 49 and 50, so x could be 49 and therefore odd. But is there any possible even value for x, a number exactly one smaller than (x + 1) where the product of the two is still 2450? There is: -50 and -49 give you the same product, and in that case x is even. So statement 2 again is critical to get the answer C.

And the lesson? A ridiculously easy statement typically holds much more value than “oh this is easy to eliminate.” So when you can quickly and effortlessly make your decision on a Data Sufficiency statement, don’t be too happy to take your slightly-increased odds of a correct answer and move on. Use that statement to give you insight into how to attack the other statement, and take your probability of a correct answer all the way up to “certain.”

*By Brian Galvin*

What does that mean?

Consider the statement:

8 = 8

That’s obviously not groundbreaking news, but it does show the underpinnings of what makes algebra “work.” When you use that equals sign, =, you’re saying that what’s on the left of that sign is the exact same value as what’s on the right hand side of that sign. 8 = 8 means “8 is the same exact value as 8.” And then as long as you do the same thing to both 8s, you’ll preserve that equality. So you could subtract 2 from both sides:

8 – 2 = 8 – 2

And you’ll arrive at another definitely-true statement:

6 = 6

And then you could divide both sides by 3:

6/3 = 6/3

And again you’ve created another true statement:

2 = 2

Because you start with a true statement, as proven by that equals sign, as long as you do the exact same thing to what’s on either side of that equals sign, the statement will remain true. So when you replace that with a different equation:

3x + 2 = 8

That’s when the equals sign really helps you. It’s saying that “3x + 2″ is the exact same value as 8. So whatever you do to that 8, as long as you do the same exact thing to the other side, the equation will remain true. Following the same steps, you can:

Subtract two from both sides:

3x = 6

Divide both sides by 3:

x = 2

And you’ve now solved for x. That’s what you’re doing with algebra. You’re taking advantage of that equality: the equals sign guarantees a true statement and allows you to do the exact same thing on either side of that sign to create additional true statements. And your goal then is to use that equals sign to strategically create a true statement that helps you to answer the question that you’re given.

**Equality applies to all terms; it cannot single out just one individual term.**

Now, where do people go wrong? The most common mistake that people make is that they don’t do the same thing to both SIDES of the inequality. Instead they do the same thing to a term on each side, but they miss a term. For example:

(3x + 5)/7 = x – 9

In order to preserve this equation and eliminate the denominator, you must multiply both SIDES by 7. You cannot multiply just the x on the right by 7 (a common mistake); instead you have to multiply everything on the right by 7 (and of course everything on the left by 7 too):

7(3x + 5)/7 = 7(x – 9)

3x + 5 = 7x – 63

Then subtract 3x from both sides to preserve the equation:

5 = 4x – 63

Then add 63 to both sides to preserve the equation:

68 = 4x

Then divide both sides by 4:

17 = x

The point being: preserving equality is what makes algebra work. When you’re multiplying or dividing in order to preserve that equality, you have to be completely equitable to both sides of the equation: you can’t single out any one term or group. If you’re multiplying both sides by 7, you have to distribute that 7 to both the x and the -9. So when you take the GMAT, do so with equality in mind. The equals sign is what allows you to solve for variables, but remember that you have to do the exact same thing to both sides.

Inequalities? Well those will just have to wait for another day.

*By Brian Galvin*