Prepping for the GRE Test of Your Quant Logic

The GRE math section is, ultimately, testing how you think. This tag applies even more aptly to a specific type of question that appears on the math section called quantitative comparison that gives you two quantities– anything from a simple number to an algebraic representation of a quantity in a given diagram. They always come with the same 4 answer choices, so you can always know what to expect. They are:

A. The quantity in Column A is greater

B. The quantity in Column B is greater

C. The two quantities are equal

D. The relationship cannot be determined from the information given

In GRE parlance, this question type is usually considered distinct from traditional, more straightforward “problem solving” questions.  There is a major difference between the two sections, and that key distinction lies in the word ‘solving’. With quantitative comparison, you want to avoid solving the problem. Instead, you want to be able to think about the problem so that you can answer the question quickly: which side, if any, is bigger?

Thinking logically, however, isn’t always easy. Though it may sound counterintuitive, the more you study a specific concept, the more likely you are to waste time on a quantitative comparison question. The likely reason is that your brain is in solve mode (after all, you’ve been doing a lot of practice), not in look-at-the-big-picture mode. When you take the latter approach, you can approach a problem in a logical fashion, instead of regurgitating a sequence of steps.

If you’ve done enough GRE series questions to the point that you can quickly and confidently add up a series of consecutive problems, you will definitely be able to answer the following question. But, will you be able to spot the quickest way of solving the problem?

Column A Column B
The sum of the multiples of 7 up until 1000 The sum of the multiples of 14 up until 1000

A. The quantity in Column A is greater

B. The quantity in Column B is greater

C. The two quantities are equal

D. The relationship cannot be determined from the information given

Again, by following the formula for adding a series of numbers that differ by a consistent amount, you can solve this problem (though it will take a couple of minutes to do so). Instead, write out the first few multiples for column A and B, and see if you notice any pattern (you may actually notice the pattern without even writing anything down).

Column A Column B
7+14+21+28+35+42 14+28+42

A. The quantity in Column A is greater

B. The quantity in Column B is greater

C. The two quantities are equal

D. The relationship cannot be determined from the information given

What do you notice? Well, every number in Column A is in Column B. However, column A also has more numbers (7, 21, 35, etc.) that are not in column B. Therefore, Column A must be greater than Column B (remember both end at a 1000…994 to be exact).

And that’s it. With just one logical breakthrough, you can save yourself over a minute.

Takeaway:

Know how to solve a problem. But, also know how to look at the big picture, especially on quantitative comparison questions. Often, you’ll find a much faster way of doing a problem without losing accuracy.

This post was written by Chris Lele, GRE and GMAT Expert at Magoosh Test Prep. Magoosh offers hundreds of practice questions and video lessons, as well as free resources and tips on how to master the GRE and GMAT. Read our GRE Study Guides and Plans to learn how to plan a study strategy for the new GRE.

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