With the winter solstice behind us here in the Northern Hemisphere, you’re probably noticing that the daylight is starting to return; this week we begin the steady climb toward summertime and you’ll see a few extra minutes of daylight after work each week from here until June. For many GMAT applicants, the darkest days of the year in December and early January match with the darkest days of their admissions journey, hustling to post a competitive GMAT while also scrambling on essays for Round 2. But this, too, shall pass.
If your New Year’s Resolution is to make 2015 the year that you ace the GMAT, you can take a lesson from this time of year. The darkest points always give way to enlightenment, and that secret will get you through some very difficult GMAT problems. There are two very common structures for challenging GMAT quant problems:
1) It looks easy, but the last step or two are tricky.
2) It looks impossible, but once you’ve found the right foothold it gets easy quickly.
This post is all about #2, those problems where it looks incredibly dark right up until that moment that you reach enlightenment. Veritas Prep’s own Jason Sun recounts the first quant question en route to his official 780 score: “I stared at a nasty sequence problem for probably 45 seconds with my jaw open thinking ‘there’s no way to solve this’. Then I remembered the strategy of starting with small numbers and finding a pattern, and 10 seconds later the answer was obvious.”
That’s common on the GMAT, and step one for you is to realize that problems are designed to look like that. When things look darkest, have faith that they’ll clear up. Here are a few ways that that occurs on the GMAT.
Calculations look awful, but work themselves out before you get to the answer.
Consider this problem:
If the product of the integers a, b, c, and d is 1,155 and if a > b > c > d > 1, then
what is the value of a – d?
Upon first glance, 1155 and four variables might look really messy. But take the first step – you know it’s divisible b y 11 and that you have to factor it. 1100 is 11*100 and 55 is 11*5, so you have 11*105. And 105 is much easier to divide out since it ends in a 5. That’s 21*5, which is 7*3*5. Once you’ve factored it down, it’s 11*7*5*3, which are all prime, so when 1 has to be less than any of these, that’s exactly a, b, c, and d. You need the biggest minus the smallest, and 11-3 is 8. What may have looked like a big, intimidating number was actually not so bad once you took the first step. It’s always darkest before the light goes on.
The problem is abstract, but comes into focus when you test small numbers.
What is the units digit of 2^40?
2^40 is an insanely large number. You’ll never be able to calculate it. But if you take the first few steps with small numbers, you’ll see a pattern:
2^1 = 2
2^2 = 4
2^3 = 8
2^4 = 16
2^5 = 32
2^6 = 64
2^7 = 128
2^8 = 256
And since you only care about the units digits, you should see a pretty firm pattern emerging. 2, 4, 8, 6, 2, 4, 8, 6. If you repeat through this pattern, you’ll see that every 4th number is a 6, and since 2^40 will be the finish of the tenth run of that every-fourth-number cycle, the answer has to be 6. The GMAT loves to give you problems with big or abstract numbers that seem unfathomable, but if you test properties with small numbers you can often find a pattern or some other way to determine what you have.
It’s always the last place you look.
Another common theme is specific to geometry problems – the GMAT often constructs them so that a seemingly irrelevant piece of information (like the measure of a far, far away angle, or the area of a figure when you’re only solving for the length of one line) is crucial to the answer…it’s just that you don’t even consider filling in that piece of information that seems so far away from what you’re really trying to solve for. So FILL IN EVERYTHING! Even if it seems irrelevant, fill in every piece of information you can solve for and you’ll give yourself a better shot of finding that unlikely relationship that cracks the code.
You’re not supposed to be able to solve for it, but you can estimate or use answer choices.
Plenty of GMAT questions beg you to do some horrifying math, but if you look at the answer choices ahead of time you can see that they’re either spread incredibly far apart and ready to be estimated or they have easy-to-plug-in properties that allow you to just test them. It’s crucial to remember that the GMAT isn’t a test of pure math, but of problem solving using math. Heed this advice: if you think the calculations are too detailed to do in two minutes, you’re probably right. That’s when you should look to estimate or backsolve.
So if your GMAT study sessions are growing longer as the daylight does, keep this wisdom in mind. It always looks darkest before sunrise, and the same is true of many tough GMAT quant problems. As you struggle through practice problems, pay attention to all those times that the solution wasn’t nearly as bad as it seemed it would have to be upon first glance.
By Brian Galvin