4 Common Types of Data Sufficiency Traps

by Joe Lucero Jul 31, 2012

If the GMAT were a sport, it would definitely be baseball, and not just because it’s three and a half hours long. In baseball, you might dominate the minor league by hitting fastballs, but once you reach the show you’ll have to hit some change-ups and curveballs too. Not only is the GMAT going to throw you some hard problems, but once you start to do well, the GMAT will throw you something different. That’s why learning the types of trap answers can help you from falling for them. Here’s four types of curveballs that you want to be mindful of on test day.

Note: the examples below are adapted from Manhattan practice tests and do not include complete explanations of the questions; rather, they are used to show examples of trap answers. If you want to learn more about these problems or others, check out the Manhattan Prep Forums, a free resource where anyone can search for and view solutions to our GMAT problems.

1)  The C Trap – United We Stand, Divided We Still Stand

For many students who are solving Data Sufficiency problems for the first time, C is a popular answer choice because students will take all information given and try to answer the question. But if you can answer a question with both statements, you have only eliminated answer choice E. That’s why setting up a grid can help to keep you focused on working with one statement at a time.

If x + 2y = z, what is the value of x?

(1) 3y = 4.5 + 1.5z

(2) y = 2

Your first instinct here might be to look at the two statements together and say, if I know y = 2, I could plug that into the first statement to solve for z, and plug y and z into the question to solve for x. And you would have an answer to the question, but you would have the wrong answer to knowing how much information is sufficient to answer the question. Note that our first equation, x + 2y = z, can be rearranged to isolate x, x = z – 2y. If we can find (z – 2y), we know what x is and that’s exactly what we can do with statement 1: 1.5z – 3y = -4.5 or z – 2y = -3. The correct answer is A.

Takeaway: If the statements together can definitely give you an answer, see if one or both statements can give you an answer by themselves.

2)  The E Trap – Impossible for You Isn’t Impossible for Everyone

If your computer doesn’t work, your first thought isn’t that it’s impossible to fix. So why do we all get to a tough Data Sufficiency problem and throw our hands in the air saying that there’s no way to solve it. Spoiler alert: in the 13th Edition of the Official Guide where problems are listed in relative order of difficulty, of the 174 DS questions, no question from 151-174 has E as the correct answer. If I ever saw a GMAT DS question written in Turkish, I would guess A-D and assume a Turkish mathematician could answer the question.

In which quadrant of the coordinate plane does the point (x, y) lie?

(1) |xy| + x|y| + |x|y + xy > 0

(2) -x < -y < |y| Everyone loves questions that include coordinate planes, absolute values, and inequalities. But rather than say there's no way that I can solve this question, if I had to guess, I would guess between A, B, C, or D. Or, instead of getting flustered, I could plug in four points from each coordinate (i.e. (1,3), (-1,3), (-1,-3), & (1,-3), and discover that only points in quadrant 1 (1,3) work with each statement. The correct answer is D. Takeaway: If a difficult quant problem looks too complex for you to solve in two minutes, assume someone else could eventually solve it. [caption id="attachment_3535" align="alignright" width="300"] The one constant through all the years, Ray, has been the GMAT. America has rolled by like an army of steamrollers. It[/caption]

3)  The 1 Looks Like 2 Trap – The Dead Ringer

If a Data Sufficiency question asked you if x = 2, the statement x < 0 would be more helpful in answering the question (answer = no) than x > 0 would be (answer = maybe). While statements may look similar, their ability to answer a given question can differ greatly.

If x, y, and z are integers greater than 1, and (3²⁷)(5¹⁰)(z) = (5⁸)(9¹⁴)(x^y), then what is the value of x?

(1) y is prime

(2) x is prime

These statements are telling us the same thing but about different variables, and that makes a world of difference. After a little canceling, the initial equation can be rephrased as 3x^y  = 25z or x^y  = 5 × 5 × z.  Since we are trying to solve for x, knowing that y is prime (or not prime) is not helpful to answer the question because z could be anything. But if x must be prime, x must be 5 because there are two fives on the right side of the equation. The correct answer is B.

Takeaway: When statements look alike, don’t assume that they mean alike.

4)  The Crucial Info Trap – The Devil is in the Details

If the GMAT tells you a piece of information, there’s usually a reason for it. But all too often, students will spend all their time doing the math and completely forget a crucial statement that the GMAT gave.

If x is a positive integer, what is x?

(1) x² + 7x – 18 = 0

(2) x² – 7x + 10 = 0

Besides being a 1 looks like 2 trap, this question has a crucial piece of information hidden at the beginning that leads to a lot of wrong answer choices: x is positive. Statement 1 tells us x could be -9 or 2, but because x must be positive, we know what x must be. Statement 2 tells us x could be 2 or 5, but both of these values are positive. The correct answer is A.

Takeaway: As you read a question, write any important information down on paper so you don’t forget them once you dive into the computations.