How to Approach Quantitative Comparison Questions in the GMAT

With regard to the GMAT, raw intellectual horsepower helps, but it is not everything. Manhattan Prep’s Stacey Koprince teaches you how to perform at your best on test day by using some common sense.

If you are taking the GRE instead of the GMAT, you will have to deal with the GRE’s “weird” question type: Quantitative Comparison (QC). What are these questions, and how do we handle them?

What is Quantitative Comparison?

The GRE and the GMAT really are not math tests, all evidence to the contrary. These tests are actually trying to test us on our “executive reasoning” skills—that is, how well we make decisions and prioritize when faced with too many things to do in too little time.

So QC questions are really about quickly analyzing some information and figuring out a relationship between two quantities. If we label the two quantities A and B, we have four possibilities:

(A) Quantity A is always bigger than Quantity B.

(B) Quantity B is always bigger than Quantity A.

(C) The two quantities are always equal.

(D) I cannot tell, or there is not an “always” relationship; maybe sometimes A is bigger and sometimes B is bigger, or sometimes A is bigger and sometimes they are equal.

We do, of course, have to do some math—and sometimes that math is quite annoying. We usually do not, however, have to do as much as we usually do on regular “problem solving” questions (the normal Quant questions).

How does Quantitative Comparison work?

First, the question is always the same: figure out the “always” relationship, if there is one (in which case the answer is A, B, or C), or figure out that there is not an “always” relationship, in which case the answer is D.

Some QC questions will provide us with “givens”—information that must be true and that we will need to use when answering the question. For example, a problem might read as follows:

x > 0

So now I know that x is positive. Is it an integer? Maybe. But it could also be a fraction or decimal, as long as that value is positive.

Next, the problem will give us two columns with their own pieces of information. For example:

Quantity A                                          Quantity B

x = 3                                                      x2-9 = 0

We do not have to do anything with Quantity A; it already tells us what x is. What about Quantity B? Solve:

(x+3)(x-3) = 0

x = -3, x = 3

It seems like the answer should be D, right? Sometimes Quantity A is bigger and sometimes they are the same. Do not forget about our “given,” though! We are only supposed to use positive values for x, so we can ignore x = -3 for Quantity B. Both quantities are always equal, so the answer is C.

Okay, these are weird. How do I get better?

These are going to take some practice, yes. In addition, this was only a very short introduction; a ton of great strategies are out there that you can learn. Look for books, articles, classes, and other resources to help. (Here is one to get you started.)

You also, of course, have to learn a bunch of math. What we have presented here, though, should help you get started on this kind-of-bizarre question type in the first place!

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