BTGModeratorVI wrote: ↑

Mon Dec 28, 2020 8:36 am

Is ab a prime number?

(1) a is a prime number.
(2) b is not a prime number.

Answer: E
Source: Veritas Prep

Kudos for a correct solution.[/quote]

Target question: Is ab a prime number?

Statement 1: a is a prime number
This statement doesn't FEEL sufficient, so I'll TEST some values.
There are several scenarios that satisfy statement 1. Here are two:
Case a: a = 2 and b = 1 , in which case ab = 2, and 2 IS prime
Case b: a = 2 and b = 6 , in which case ab = 12, and 12 in NOT prime
Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

Aside: For more on this idea of plugging in values when a statement doesn't feel sufficient, you can read my article: https://www.gmatprepnow.com/articles/dat ... lug-values

Statement 2: b is not a prime number
This statement doesn't FEEL sufficient, so I'll TEST some values.
There are several scenarios that satisfy statement 2. Here are two:
Case a: a = 2 and b = 1 , in which case ab = 2, and 2 IS prime
Case b: a = 2 and b = 6 , in which case ab = 12, and 12 in NOT prime
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Statements 1 and 2 combined
There are still several possible scenarios that meet both statements. Here are two:
Case a: a = 2 and b = 1 , in which case ab = 2, and 2 IS prime
Case b: a = 2 and b = 6 , in which case ab = 12, and 12 in NOT prime

Since we cannot answer the target question with certainty, the combined statements are NOT SUFFICIENT

Answer = E

Cheers,
Brent