Data Sufficiency

BTGmoderatorDC wrote: ↑

Sun Apr 17, 2022 8:15 pm

Are the integers z and f to the right of 0 on the number line?

(1) The product of z and f is positive.
(2) The sum of z and f is positive.


OA C

Source: GMAT Prep

Target question: Are the integers z and f to the right of 0 on the number line?
This is a good candidate for rephrasing the target question.

If z and f to the right of 0 on the number line, then that means z and f are both POSITIVE
Let's now REPHRASE the target question....
REPHRASED target question: Are integers z and f both POSITIVE?

Aside: Here’s a video with tips on rephrasing the target question: https://www.gmatprepnow.com/module/gmat- ... cy?id=1100

Statement 1: The product of z and f is positive.
There are two possible scenarios that satisfy statement 1:
Case a: z and f are both positive. In this case, the answer to the REPHRASED target question is YES, z and f are both POSITIVE
Case b: z and f are both negative. In this case, the answer to the REPHRASED target question is NO, z and f are not both POSITIVE
Since we cannot answer the REPHRASED target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: The sum of z and f is positive.
There are infinitely-many values of z and f that satisfy statement 2. Here are two:
Case a: z = 1 and f = 1. In this case, the answer to the REPHRASED target question is YES, z and f are both POSITIVE
Case b: z = -1 and f = 2. In this case, the answer to the REPHRASED target question is NO, z and f are not both POSITIVE
Since we cannot answer the REPHRASED target question with certainty, statement 2 is NOT SUFFICIENT

Statements 1 and 2 combined
From statement 1, we concluded that EITHER z and f are both positive OR z and f are both negative
If z and f are both positive, then the sum of z and f will be POSITIVE
If z and f are both negative, then the sum of z and f will be NEGATIVE

Statement 2 tells us that The sum of z and f is positive.
AHA!
This means it must be the case that z and f are both positive
So, the answer to the REPHRASED target question is YES, z and f are both POSITIVE
Since we can answer the REPHRASED target question with certainty, the combined statements are SUFFICIENT

Answer: C

Cheers,
Brent