Simple Quantitative Strategies for the GMAT

I can’t tell you how many times I’ve seen a question that requires you to solve for some unknown variable x, and then it asks you “What’s the value of x/2?” or “What’s the value of 4x” or “What’s the value of sqrt(x)?”

The test writers know that multiplying your final answer by 2 does not prove your mathematical prowess. Rather, it tests your test taking ability.

Often, what happens is that students read a question, figure out what they must do to arrive at the answer (say, solving for variable x), and then stop once they’ve figured out said variable. Why go any further? I’ve just done the algebra. I’ve unlocked the problem; I’ve got the answer. It’s that feeling of knowing the familiar process of solving for x that is dangerous; once you arrive at x, you feel finished. And, I guarantee that the value of ‘x’ will be in the answer choice, further reassuring you that you’ve completed this question correctly. But, unfortunately, the answer is not x, but 4x (or some variation, arbitrary or not, invented by the test writers).

To avoid this, consciously think about following directions more so than you usually do. Don’t assume you know the drill, even if you really do know the drill. The test writers are looking for these cheap ways to trick smart students into missing the answer.

To ‘ballpark’ is to roughly approximate. In terms of quantitative strategy, ballparking essentially means thinking about mathematical figures in a vague, imprecise, but nonetheless common sense manner. When we are overwhelmed by figures and calculations, it’s easy to make mistakes. Moving a decimal one unit could transform a correct answer into a wrong answer, no matter how many correct steps you painstakingly went through. This is where ballparking plays a significant role. Let’s look at a crude example:

What’s 32.33 % of 50?
A. 5.125
B.16.165
C. 35.685
D.50.350
E. 70.195

Any relaxed, common sense thinker will probably be able to answer this question without doing a calculation. But, when you’re in the middle of a timed test, things change. Anxiety sets in, and you go into human calculator mode. You see a question like this and immediately start calculating the product.

Step back, though, and look at the simplicity of the question. 32.33 percent is awfully close to one third. A third of 50 is a little more than 15 (15*3= 45). The only thing close to that is B; it can be no other answer.

Nearly every multiple choice math problem has trap answers, or attractors. These types of answers catch your eye for one reason or another, often making the problem appear a little bit easier than it actually is. You may notice the anticipated answer in the choices and think that you won’t have to finish the problem, but remember that such a choice is probably a trap. Looks look at an example of what this might look like:

The price of a T-shirt was reduced by 20%. Then, during a special sale, the price was reduced another 20%. What was the total percentage discount from the original price?
a. 25%
b. 36%
c. 40%
d. 42%
e. 50%

You may read this question and think that a 20 percent discount plus another 20% discount equals a 40% discount. Seeing 40% as an answer choice, you may be inclined to choose it and move on. Unfortunately, you’ve just missed a pretty easy question. Did you really think that the test would give you a question that required such minimal effort as adding 10 and 10? It’s nice to dream, isn’t it? But, let’s get real. Just perform the calculations as necessary.

First, why not imagine the shirt is 100 bucks to start.

Take 20% off of 100, and you get 80.

Take 20% off of 80 (80 / 5 = 16) and you get 64.

We went from a 100 dollar shirt to a 64 dollar shirt. That’s a difference of 100-64=36.

Thus, the total discount is $36, B.