Two Devious GMAT Math Traps and How to Avoid Them

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Two Devious GMAT Math Traps and How to Avoid Them

Don't fall for these two common GMAT trapsIncreasing your Quantitative score on the GMAT can present a formidable challenge, especially if math is not your strong point.  Not only does the GMAT cover a wide range of mathematical concepts, but it also engineers many of its questions to make incorrect answers appear like the correct solution.

The exam’s writers love to insert tempting false answers that play off common human fallibilities, like a momentary lack of concentration. However, with a little practice, you can learn how to spot these traps with ease.

The GMAT will try to deceive you

The GMAT is designed to produce a standard distribution of test scores, meaning that while some people will score very high on the test and some very low, most test takers will score in the middle range.  To produce this distribution, the exam’s writers employ a number of tactics to ensure that the majority of test takers will make mistakes.

One of these tactics is to create traps in questions that most test takers will fall for.  A key then to increasing your Quantitative score, or your GMAT score in general, is to avoid these traps the GMAT will throw at you and thus separate yourself form the majority of test takers.  This skill is more about being aware of how the GMAT writes its questions than learning any new math concepts. Let’s examine two of the most common traps the GMAT uses.

1) The answer that’s too good to be true

When you’re working your way through the Quantitative section, you’re bound to come across more than a few questions where an answer choice jumps out at you before you’ve even worked through the math.  An answer that looks too good to be true often isn’t.

The GMAT is a tough test, so if you see an answer that looks obvious and requires little to no thinking to arrive at, resist the urge to quickly select it and move on.  Instead, take a closer look at the question.  Often, the math involved is not as straightforward as it first appears.  Take the following example:

In the first week of January, a certain stock was purchased for x dollars.  The next week, the stock’s price dropped 15%, and the following week, the stock’s priced dropped an additional 10% from the previous week’s price.  By what percent must the stock’s price be increased in order to reach its original value of the first week?   

A) 30.7%

B) 23.5%

C) 25%

D) 40.5%

E) 12%

The writers of the GMAT are banking on the fact that the average test taker will select answer C, an answer that seems to make intuitive sense.  A decrease of 15% followed by a decrease of 10% should add to a total decrease of 25%, right?  However, few if any Quantitative questions are that easy.  Don’t be fooled into thinking you “lucked out” with a simple question.

The key piece to the question above, which the “obvious” answer obscures, is that the second percent decrease is taken off of the value that resulted from the first percent decrease and not the original value of the stock.  Let’s work through the problem.

When presented with an unknown quantity in an a question like this, plug in a number of your choosing.  For questions that deal with percent change, the number 100 works best.

If we take $100 for the stock’s original price, then a 15% decrease would equal $85.  A further 10% decrease would come off of this 85 and not the original $100.  Ten percent of 85 is 8.5, so a 10% decrease equals 85 – 8.5 or 76.5.  The decreased value of $76.50 would need to increase $23.50 to equal the original value of $100, and 23.5 represents 30.7% of 76.5.

Once again, what first appears as mindless math actual covers up a more complex calculation.  Don’t trust the easy answer.

2) The right answer to the wrong question

Being wary of an obvious answer will help you to steer clear of some mistakes, but the GMAT can be much more subtle in how it lays its traps.  Another popular tactic is to put an almost correct answer into the answer selection.  What this means is that an answer bank will often include a value that represents one part of the solution but not the final answer.  Look at the example below:

If Philippe can paint 4 walls every 48 minutes and Jerry can paint 3 walls every 30 minutes, how many more walls will the faster painter paint in a span of 3 hours? 

A) 18

B) 7

C) 1

D) 10

E) 3

This question does not contain an obvious, no-thinker answer like the last example, and though there’s some calculations involved, the question is fairly straightforward.  This is a rate/work problem, and because the total time is in hours, it makes sense to first calculate each painter’s rate in hours.

Jerry’s rate in hours is simple: 3 walls every 30 minutes equals 6 walls every hour.  Philippe’s rate is a little more complex; however, if you recognize that 48 minutes is 4/5ths of an hour, then you know that he can paint 5 walls in an hour.

Given this information, you can easily calculate that in 3 hours Philippe will have painted 15 walls and Jerry will have painted 18.  If you had read through the question quickly, you may be tempted to stop here and select A since the faster painter painted 18 walls.  However, the question asked how many more walls the faster painter painted, so it’s asking for the difference between the two painters, which is answer E.

The likelihood of falling for this trap increases as the complexity of the question increases.  It’s easy to become caught up in your calculations, especially if you’re racing the clock.  When you finally reach that “aha” moment when you find a solution, you may not notice that question asked for something different or that there’s one more step you forgot to do.

To avoid making this mistake always refer back to what specifically the question is asking.

Stay alert and outwit the test

Always be aware that the GMAT will try to trick you with tempting false answers, especially on math questions where you can become easily distracted. By keeping your mental guard up and knowing how the GMAT will try to deceive you, you can take a large step forward towards reaching your goal score

1 Comment

  1. Fady Ibrahim says:

    I think the example in this tip is incorrect. It says Phillipe will have painted 15 walls…but 3 walls each half hour means 18 walls in 3 hours.

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