Tales from the GMAT Question Bank: When You Assume You Make... A Big Mistake

This blog post is one in a series of lessons that come from the free Veritas Prep GMAT Question Bank and the statistics gathered from its user base. For each question, the data behind correct and incorrect answers tell a story, and many of these stories hold in them great value for you as you prepare to take the GMAT. In each of these posts, we’ll take a question from the GMAT Question Bank and show you what you can learn from the trend in correct/incorrect answers submitted by other students.

When Veritas Prep hosts its free seminars online — 1.5 hour sessions that introduce prospective students to the GMAT and to several strategies for succeeding on the test, as well as introducing them to the Veritas Prep program — one of the first items that the presenter covers is a Data Sufficiency question that highlights the GMAT “penalty” for making assumptions about numbers. Through that demonstration, students quickly realize their own propensity for thinking in terms of positive integers, and are taught to write down a quick checklist to ensure that they consider both negative numbers and nonintegers.

With that in mind, then, you can make a strong case for the fact that the trap embedded in this question is “Page One” for prospective test takers. Take a look and see if you can spot the trap:

If xyz = 1001, is x odd?

(1) y is odd
(2) z is odd

Now, before we reveal the answer choices and choice-by-choice statistics from Question Bank users, let’s point this out: this question ranks as one of the top 30 most difficult questions in the bank, with only 15% of all respondents answering correctly (and, as you’ll see, a sample size of over 200 users). And consider this: in GMAT multiple choice questions a random guess has a probability of 20% of being correct. Question Bank users – those who care enough about their GMAT progress that they are actively seeking out additional practice questions – are worse than a random guess at this question. Which should go to show you the power of the trap being laid here. Let’s go to the statistics, taken straight from the GMAT Question Bank:

Note the answers – almost no one thought that either of statement 1 or 2 (but not both) was sufficient, which is good evidence that just about everyone in the question bank “tried” on this question. And the most popular answer choices were just about split between C and D. In either case, people employed the number property that Odd*Odd*Odd = Odd – without considering that x could be a noninteger. Consider this possibility:

y = 1001, z = 1001, and x = 1/1001.

The statements hold, but x is not odd in this case.

What’s even worse is the procedure that those who spent more time on this might have used. It’s possible to factor out 1001 into 7 * 11 * 13, but that’s a time-consuming process. So for those who investigated that much further, they may well have gotten this question wrong in over two minutes, costing them not just one correct answer but some valuable time en route to another.
What can you learn from this? A few things:

  1. Don’t be overly impressed by seeing opportunities to employ rules that you have memorized. It’s quite easy to get lulled into a wrong answer because the question “rewarded” you for knowing something that you had on a flash card, but keep in mind that this is a reasoning test that will force you to think often. When an answer seems too good to be true, investigate further.
  2. It’s “Page One,” but be absolutely certain not to make assumptions on Data Sufficiency questions. Force yourself to consider numbers with different properties – negatives, nonintegers, and 0 in particular.
  3. Don’t assume that, just because a question looks easy, it’s easy. This question is in no way one of the top 30 “most intimidating” questions in the question bank, but it ranks as one of the 30 most frequently missed.

At Veritas Prep, we’re big fans of the strategy “Think Like the Testmaker” — meaning, learn how the authors of the GMAT employ devices to trap you. While it’s likely one of the first traps you learn about, the assumption that we all make about integers or positive numbers frequently lends itself to trap answers. As you can see from these statistics, many elite test-takers missed this question because of that assumption. And as you’ll see in future posts (and, although we hope not, probably in your own future mistakes) these types of mistakes happen to just about everyone and make for some of the statistically-most-difficult questions on the GMAT. Be careful about assumptions — and don’t just take our word for, it, but rather listen to almost 85% of your GMAT competitors who had to learn this one the hard way.

Go ahead and see how you do on the GMAT Question Bank, an entirely free source of hundreds of realistic GMAT questions!

Plan on taking the GMAT soon? We run a free online GMAT prep seminar every couple of weeks. And, be sure to find us on Facebook and Google+, and follow us on Twitter!

By Brian Galvin

4 Responses

  1. Nate Martin says:

    Important typo, text:

    If xyz = 1001, is x odd?

    (1) y is odd
    (2) x is odd

    looks like it should be (according to screenshot)

    If xyz = 1001, is x odd?

    (1) y is odd
    (2) z is odd

  2. Domenico says:

    hi brian fantastic inside.

    however two things: I think 2) stat is Z and not X as I can see on the screenshoot. Secondly, maybe is more correct to say that 1/1001 is neither odd nor even because odd even is applied only to integers.

    For the record, amazing as usual from you :)

    • Brian says:

      Thanks, Domenico – and sorry about that x vs. y deal (corrected that…Nate caught it too). That was just boneheaded on my part – I was trying to type up the question from memory without the original or graph in front of me. And you’re right about 1/1001 – I think I have it as “x is not odd” in the post, but good to clarify that it’s not even, either.

      Glad you liked this one – we’re going to make “Tales from the Question Bank” a recurring weekly feature on here so stay tuned for more!

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