Which of the following are/is prime?

BTGmoderatorDC wrote: ↑

Sat Apr 10, 2021 10:09 pm

Which of the following are/is prime?

I. 143
II. 147
III. 149

(A) II only
(B) III only
(C) I & II
(D) I & III
(E) I, II, & III
Solution:

143 is not prime since 143 = 11 x 13, and 147 is not prime since 147 = 7 x 21. However, 149 is prime since its only factors are 1 and 149.

Alternate Solution:

We should immediately realize that 147 is not prime since it is divisible by 3 (which we can tell from the fact that the sum of the digits of 147 is divisible by 3). Thus, we eliminate answer choices A, C and E. To decide between B and D, we only need to determine whether 143 is prime. Since 143 = 11 x 13, it is not prime, and answer choice D is also eliminated. The only remaining answer choice is B.

Answer: B


OA B

Source: Magoosh