If Set A is made up of all 2-digit primes that are one less than a power of 2, what is the standard deviation of set A?

[BTGmoderatorDC]

If Set A is made up of all 2-digit primes that are one less than a power of 2, what is the standard deviation of set A?

A. 0
B. 15.1
C. 17
D. 20
E. 23.7


OA A

Source: Veritas Prep

[Brent@GMATPrepNow]

If Set A is made up of all 2-digit primes that are one less than a power of 2, what is the standard deviation of set A?

A. 0
B. 15.1
C. 17
D. 20
E. 23.7


OA A

Source: Veritas Prep

Key concept: A set consisting of 1 value will have a standard deviation of 0

Let's list all of the 2-digit powers of 2 and subtract 1 from each value.

2⁴ - 1 = 16 - 1 = 15 NOT PRIME
2⁵ - 1 = 32 - 1 = 31 PRIME
2⁶ - 1 = 64 - 1 = 63 NOT PRIME

So, Set A = {31}, which means but standard deviation of set A = 0

Answer: A