A common mantra heard when studying for the GMAT is that you have to be fast when answering questions. This is absolutely true, as the exam is testing not only your reasoning skills but also your time management skills. This does not, however, necessarily mean that you must solve every question quickly. Indeed, there may be times where you feel fairly confident in the answer choice you’ve selected, but you don’t feel 100% certain (maybe a strong 60%). In these situations, it’s perfectly acceptable to double check your answer manually.

Needless to say, having a sound understanding of the theory and logic of a question is ideal. Completely understanding the possibilities, rules and potential traps of a certain topic regularly leads you to select the correct answer choice. However, it is almost inevitable that a topic, notion or concept will come up that you don’t fully comprehend (or comprehend at all). In that case, it’s often best to try and determine a logical answer and double check it with some manual verification.

Obviously, if an answer asks you to sum all the integers from 1 to 150, you hopefully have a better strategy than simple brute force. Solving such a question without a calculator in less than 2 minutes is a fool’s errand. If you begin adding 1 to 2 to 3 to 4, you know you’re in trouble (unless you’re 5½ years old). Nonetheless, many questions can be solved via brute force within the given time constraints, if only with the help of a little bit of logic to narrow down the answer choices.

Let’s look at a Data Sufficiency problem that highlights these issues:

If P and Q represent the hundreds and tens digits, respectively, in the four-digit number x=8PQ2, is x divisible by 8?

(1) P = 4

(2) Q = 0

(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 but neither statement is sufficient alone.

(D) Each statement alone is sufficient to answer the question.

(E) Statements 1 and 2 are not sufficient to answer the question asked and additional data is needed to answer the statements.

This is a fairly straight forward divisibility question asking about whether a certain number is divisible by 8. However, there is one caveat: two of the digits can change. This question allows for different tens and hundreds digits, and this oscillation allows for no fewer than 100 distinct options to consider for divisibility. A brute force approach would take far too long, so we need to undertake a logical approach to a divisibility rule that is often overlooked because it is uncommon (as opposed to mythic rare).

To be divisible by 8, the rule you might know is that the last 3 digits must be divisible by 8. This essentially truncates anything bigger than the hundreds, and is due to the fact that 1,000 is divisible by 8, so any multiple of 1,000 can be ignored as it is necessarily also divisible by 8. Knowing this, we can ignore the “8” at the beginning of the number and concentrate on the 3-digit PQ2. Determining the divisibility of the last 3 digits isn’t too hard if those numbers are static. If they vary, though, the answer may be harder to pinpoint.

Let’s start with statement 1: P=4. If this is true, then we’ve turned the abstract question into the more straight forward determination of whether 4Q2 is divisible by 8, which really is just asking if {402, 412, 422, …, 492} are all divisible by 8. This is small enough that we can brute force it, especially if we recognize that 400 is divisible by 8 (the quotient would be exactly 50). 402 is then logically not divisible by 8, since it is only 2 away from a known multiple of 8. 412 is similarly not divisible by 8, and neither is 422. However, 432 is divisible by 8 (yielding a quotient of 54). This means that we have at least one value that is divisible by 8 (432) and at least one value that is not (402). Statement 1 will thus be insufficient.

Logically, this inconsistency should make sense. We are taking an even number and adding 10 to it. While 10 is not divisible by 8, multiples of 10 will be divisible by 8, and we’ll eventually cycle through a few numbers that are perfectly divisible by 8. Even if we can’t easily see this logic on test day, a strategic brute force will confirm these suspicions. There are only 10 numbers to check in the worst case, and we can stop whenever we can confidently say whether the statement is sufficient or not. This leaves only answer choices B, C and E possible.

Let us now look at statement 2: Q = 0. This ultimately means we must check the divisibility of P02, which is {102, 202, 302, …, 902}. This is not necessarily trivial, but if we check for 102, we know that 80 is divisible by 8, thus so is 88, 96 and 104. Since 102 falls in the gap between two multiples, it is not a multiple of 8. Next we can check 202, and if you recognize that 200 is a multiple of 8 (8×25), you’ll know fairly quickly that 202 is not a multiple. You can check the remaining eight choices quickly if you use your logic and start from numbers you know to be divisible by 8 (400, 600, 800). Even using this painstaking method, you can determine that all ten choices are not divisible by 8 within a minute. If none of the choices work, then we can confidently assert that this statement is sufficient to get a consistent answer of no on this question. Answer choice B is correct.

There are more logical tenets that help guide you on these types of questions, but they’re not necessarily well known. For example, any number that is divisible by 8 must also be divisible by 4, meaning that dividing by 4 can be used as an easy filter (like coloring inside the lines). Any number that ends in 02 will not be divisible by 4, no matter what the hundreds digit is. Therefore, this statement will always produce an answer of no. Even if you utilized this property and were leaning towards answer choice B, it doesn’t hurt to double check your answers manually. Often, double checking your answers can lead to double digit improvements on your GMAT score.

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*Ron Awad is a GMAT instructor for Veritas Prep based in Montreal, bringing you weekly advice for success on your exam. After graduating from McGill and receiving his MBA from Concordia, Ron started teaching GMAT prep and his Veritas Prep students have given him rave reviews ever since.*