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Doing Well on Quantitative Comparison Questions Even When You Have No Clue

  • by John
  • Jun 10, 2011
  • GRE Blog

You have to guess on every single GRE problem– you won’t be able to continue with the test until you’ve chosen an answer. So you might as well learn to guess well. On Quantitative Comparison questions, as I’ll show, this is especially, almost scandalously easy.

Let’s take a made-up QC question:
-1 < x < y < 1
x does not equal 0.
y does not equal 0.

Column A: x / y
Column B: y / x

A. Column A is larger.
B. Column B is larger.
C. The two quantities are equal.
D. Cannot be determined from the information given.

You have four choices; assuming you knew no math whatsoever, your odds of getting the question right would be 25%. But we can up those odds substantially with a simple trick:

Plug in Numbers

Let’s choose two numbers that satisfy the conditions. How about:
x = -0.5
y = 0.5

Now, let’s test them out. We get:

Column A: x / y = -0.5 / 0.5 = -1
Column B: y / x = 0.5 / -0.5 = -1

What we have discovered is that for the number we chose, the two quantities are equal. Now, we can’t be sure just from this that the answer is C: it may be that the two quantities are sometimes equal and sometimes not, which would indicate answer choice D. But we can be sure that neither Column A nor Column B is always larger, so we can eliminate both A and B. Just like that, without so much as thinking about it, we have narrowed the choices down to C and D and doubled our odds of guessing correctly; we now have a 50% chance.

If you’re unsure of how to attack a quantitative comparison question, the first thing you should do is plug in numbers. You will always be able to eliminate some answer choices very quickly this way.

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