On the GMAT, Data Sufficiency questions can be tricky. But perhaps most frustrating about Data Sufficiency questions are those that somehow trick you when, upon further review, they gave you absolutely everything you needed. When you look back at them, you can’t believe that you got them wrong – but you should also notice patterns in why you did. One common way that an in-hindsight-pretty-straightforward question can be extremely challenging involves the “hiding” of pertinent information in the question stem itself, where the testmakers know that you’re apt to read quickly in your haste to get to the statements. Consider the question:

If xy < 0, is x/y > z?

(1) xyz < 0 (2) x > yz

One of the major keys to solving this problem is to fully digest the initial fact: xy < 0. This tells you that one of x and y is negative and the other is positive, and when you combine that with statement 1 you learn that "when a negative number xy is multiplied by z, it stays negative". This means that z has to be positive. The given information also tells you that x/y is negative, because you know that x and y have different signs. So by fully unpacking the given information along with statement 1, you know that:
z is positive
x/y is negative

So the statement is sufficient – the negative number x/y cannot be greater than the positive number z.

Statement 2, on the other hand, is not sufficient. You know from the given information that either x or y is negative, but you don’t know which one. So you cannot simply divide both sides of the statement 2 equation to mirror the question, because you know that there’s a 50% chance that y is negative and in that case you’d have to flip the sign. So the answer is A, and the important lesson is that you need to leverage the information in the question stem on many problems in order to fully understand the problem.

The GMAT knows that we tend to rush through the question stem so that we can get to work on the statement, so many difficult questions are constructed so that they reward those who fully leverage the question stem as an asset and not just as a “backstory.” The GMAT knows that it can hide crucial information in plain sight, so be certain to use the question stem to your advantage.

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3 Responses

1. domenico says:

this was quite tough………..I pick it in 1.30 (the most time dedicated to an upfront evaluation).

I’m pretty confident now after months of study on this kind of problem. what is the level ?? at least >= 650 ??

Thanks to your work for us with this amazing blog

2. Brian says:

Thanks, Domenico! I think it’s fair to say that this is above average difficulty. I actually just wrote it this morning for this post, kind of inspired by a few problems I’ve seen recently, so I haven’t seen enough people try to solve it yet to really know just how tough it is, but the goal was for it to be in the 600s or so…

3. domenico says:

two useful tips are those:

when we have questions with > or < zero is quite certain the we deal with signs (plus or minus).

when we have a question that starts with IF in the stimulus is always good to evaluate the statements in conjunction with IF i.e: for 1) we have xyz < 0 AND IF xy < 0…………

regards :)