If Set A is made up of all 2-digit primes that are one less than a pow

Question

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

houston1980 · Answer

31 is the ONLY 2-digit primes that is one less than a power of 2.

Therefore set A contains only the number 31. Therefore the standard deviation is zero (0).

Hence   option A    is the answer.

onlynk1 · Answer

Set A ={31,63} Mean= 47 If we go with proper definition and calculation SD={-16,+16} ={0/2} =0 Option A ?

houston1980 · Answer

Hello  onlynk1

The question says : 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?

63 is NOT a prime.   . Thanks.

ScottTargetTestPrep · Answer

The only 2-digit powers of 2 are:

16, 32, 64

If we subtract one from each of these numbers, we have:

15, 31, 63

Since the only prime number is 31, set A = {31}. Lastly, since there is only 1 number in the set, the standard deviation is zero.

Answer: A

push12345 · Answer

Only 31 qualify for condition above

Mean =31 for set A
Standard deviation calculation includes difference between mean and number itself
Since only one number is present so it's mean only
Hence SD =0

Posted from my mobile device

Basshead · Answer

Only two digit powers of 2: 16, 32, 64

One less than a power of 2: 15, 31, 63

Only prime = 31.

SD = 0. Answer is A.

ScottTargetTestPrep · Answer

Solution:
 
The two-digit numbers that are powers of 2 are 16, 32, and 64. If we subtract 1 from each of these numbers, we have 15, 31, and 63. Of these 3 numbers, only 31 is a prime. Therefore, set A contains only the number 31, and if a set only has one number, its standard deviation is 0.

Answer: A

Kinshook · Answer

Asked: 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 = {31}
Standard deviation of A = 0

IMO A

BrentGMATPrepNow · Answer

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

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If Set A is made up of all 2-digit primes that are one less than a pow

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