GMAT Tip: The Usual Suspects for Quant Mistakes

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This tip on improving your GMAT score was provided by Brian Galvin at Veritas Prep.

There are a few types of wrong answers on the GMAT, and one should make you exponentially angrier than the others. Some questions are just hard, ones that you can’t come up with a method to solve. Then there are those that are frustrating, but in the end you were “beaten by a better opponent,” and you can go back to those questions and learn what you need to study and how you need to improve.

The worst mistakes, though, are those in which you beat yourself—those you knew how to solve, did the work right, and still got them wrong because you made common, silly mistakes. Those are points that belong to you but that you gave back to the test by working too hastily or not paying enough attention. Those are the questions that should frustrate you the most, but on the plus side, those should be the easiest mistakes to fix.

You should be on the lookout for these mistakes and have a quick mental checklist to help you ensure that you don’t accidentally submit an incorrect answer on a question you should have gotten right. Among the most common “silly” quant mistakes are:

Answering the wrong question

If a question asks, “how many gallons are left in the bucket after 40 percent is removed?” or “If x + y  = 7 and x – y = 1, what is the value of y?”, your math will almost always lead you to solve first for the number of gallons removed or the value of x. You will end up with a number on your noteboard that matches an answer choice—but that represents the right answer to the wrong question. When there are two variables or entities in a problem, or when multiple intermediate steps are required to solve it, there’s nearly always an incorrect answer choice that matches the other variable or the number you get one step before the problem is finished. Before you submit your answer, double-check that it’s the answer to the proper question.

Forgetting to consider “unique” numbers

Particularly in Data Sufficiency questions, standard counting numbers such as 2, 3, 4, etc., will react one way, but the less-obvious numbers such as fractions, negatives, and zero may behave another. Often in your haste to answer the question quickly, you forget to consider a type of number that would change the answer completely. For example, if a question asks “Is x > 3?” and you’re given the information that x2 = 25, you might quickly think “x = 5, so yes.” But what about -5? That’s a less obvious number to consider—your mind isn’t that calibrated to think about negatives—but it changes the answer, as just given that x2 = 25, we don’t know whether x is greater than 3.

And while that’s a relatively basic example, remember this: The GMAT is incredibly adept at getting you to make this exact mistake (forgetting to consider negatives) on tricky questions. Questions that involve a clever math step or two often get you to let down your guard, so even the highest-scoring students need to be aware of these traps.

Thinking that “no” means “not sufficient”

Also in Data Sufficiency questions, you’re asked to determine when you have enough information (when the data are sufficient) to answer a question. But since your standardized-test mind is calibrated for the process of elimination, when those data are sufficient to give the direct answer “no,” you’re prone to cross wires and eliminate that answer. For example, if a question asks, “Is x a prime number?” and you’re given a statement “x = 17! + 51,” you should see that you have sufficient information to answer the question “no,” since both terms being added are multiples of 17, then x will be a multiple of 17 and therefore not prime. But your mind might say “no, it’s not prime … so NO on that statement,” when really you do have sufficient information.

Your job is to determine whether the information is sufficient, not whether the answer is “yes.”

These are three very common families of mistakes, so you should certainly be on the lookout for these, but everyone has his or her own blind spots, too. So as you make mistakes in practice, note them and pay particular attention when you make the same mistake more than once. If you draw 7s that look like 4s or you transpose the variables in rate problems, take note and remind yourself to slow down when those circumstances are likely. Much of your GMAT success will come not from “learning more things” but from protecting points that should already be yours, were it not for silly mistakes.

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