GMAT Tip of the Week: Common Quant Mistakes That You Must Avoid

Much of your GMAT preparation will focus on “more” – learning more content, memorizing more rules, feeling more comfortable with the test format, and ultimately getting more questions right. But might impact your score more than “more” is your emphasis on “less” (or “fewer”). Feeling less anxiety, taking less time on tricky problems, having to guess less than in your previous attempts, and this ever-important concept:

Making fewer mistakes.

On an adaptive test like the GMAT, making silly mistakes on problems that you should get right can be devastating to your score. Not only do you get that question wrong, but now you’re being served easier questions subsequent to that, with an even more heightened necessity of avoiding silly mistakes there. So you should make a point to notice the mistakes you make on practice tests so that you’re careful not to make them again. Particularly under timed pressure in a high-stress environment we’re all susceptible to making mistakes. Here are 5 of the most common so that you can focus on making fewer of these:

1) Forgetting about “unique” numbers.

If someone asked you to pick a number 1-10, you might pick 5 or 6, or maybe you’d shoot high and pick 9 or low and pick 2. But you probably wouldn’t respond with 9.99 or 3 and 1/3. We tend to think in terms of integers unless told otherwise. Similarly, if someone asked “what number, squared, gives you 25″ you’d immediately think of 5, but it might take a second to think of -5. We tend to think in terms of positive numbers unless told otherwise.

On the GMAT, a major concept you’ll be tested on is your ability to consider all relevant options (an important skill in business). So before you lock in your answer, ask yourself whether you considered: positive numbers (which you naturally will), negative numbers, fractions/nonintegers, zero, the biggest number they’d let you use, and the smallest number they’d let you use.

2) Answering the wrong question.

An easy way for the GMAT testmaker to chalk up a few more incorrect answers on the problem is to include an extra valuable or an extra step. For example, if a problem asked:

Given that x + y = 8 and that x – y = 2, what is the value of y?

You might quickly use the elimination method for systems of equations, stacking the equations and adding them together:

x + y = 8

x – y = 2

2x = 10

x = 5

But before you pick “5” as your answer, reconsider the question – they made it convenient to solve for x, but then asked about y. And in doing so, they baited several test-takers into picking 5 when the answer is 3. Make sure you always ask yourself whether you’ve answered the right question!

Check more in PART II

3) Multiplying/dividing variables across inequalities.

By the time you take the test you should realize that if you multiply or divide both sides of an inequality by a NEGATIVE number, you have to flip the sign. -x > 5 would then become x < -5. But the testmakers also know that you’re often trained mentally to only employ that rule when you see the negative sign, –

To exploit that, they may get you with a Data Sufficiency question like:

Is a > 5b?

(1) a/b > 5

And many people will simply multiply both sides of statement 1’s equation by b and get to an ‘exact’ answer: a > 5b. But wait! Since you don’t know whether b is positive or negative, you cannot perform that operation because you don’t know whether you have to flip the sign. When you see variables and inequalities, make sure you know whether the variables are negative or positive!

4) Falling in love with the figure.

On geometry questions, you can only rely on the figure’s dimensions as fairly-reliable measurements if: One, it’s a Problem Solving question (you can never bring in anything not explicitly provided on a DS problem); and, two, if the figure does not say “not drawn to scale”. But if it’s a Data Sufficiency problem *or* if the figure says not drawn to scale, you have to consider various ways that the angles and shapes could be drawn. Often times people will see a “standard” triangle with all angles relatively similar in measure (around 60 degrees, give or take a few), and then base all of their assumptions on their scratchwork triangle of the same dimensions. But wait – if you’re not told that one of the angles could be, say, 175 degrees, you could be dealing with a triangle that’s very different from the one on the screen or the one on your scratchwork. Don’t get too beholden to the first figure you see or draw – consider all the options that aren’t prohibited by the problem.

5) Forgetting that a definitive “no” answer to a Data Sufficiency question means “sufficient.”

Say you saw the Data Sufficiency prompt:

Is x a prime number?

1) x = 10! + y, where y is an integer such that 1 < y < 10

Mathematically, you should see that since every possible value of y is a number that’s already contained within 10 * 9 * 8 * 7 * 6 * 5 * 4 * 3 * 2 * 1, whatever y is the new number x will continue to be divisible by. For example, if y = 7, then you’re taking 10!, a multiple of 7, and adding another 7 to it, so the new number will be a multiple of 7.

Therefore, x is not a prime number, so the answer is “no.” But here’s where your mind can play tricks on you. If you see that “NO” and in your mind associate that with “Statement 1 — NO”, you might eliminate statement 1 when really statement 1 *is* sufficient. You can guarantee that answer that x is not prime, so even though the answer to the question is “no” the statement itself is “positive” in that it’s sufficient.

So be careful here – if you get a definitive “NO” answer to a statement, don’t cross it out or eliminate it!

Remember, a crucial part of your GMAT study plan should be making fewer mistakes. While you’re right to seek out more information, more practice problems, and more skills, “fewer” is just as important on a test like this. Make fewer of the mistakes above, and your score will take you more places.