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

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New post 13 Apr 2018, 03:08
1
00:00
A
B
C
D
E

Difficulty:

  45% (medium)

Question Stats:

70% (01:01) correct 30% (01:47) wrong based on 63 sessions

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Re: If Set A is made up of all 2-digit primes that are one less than a pow  [#permalink]

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New post 13 Apr 2018, 04:54
1
1
Bunuel wrote:
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





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.
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Re: If Set A is made up of all 2-digit primes that are one less than a pow  [#permalink]

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New post 13 Apr 2018, 05:06
Set A ={31,63}

Mean= 47
If we go with proper definition and calculation

SD={-16,+16}
={0/2}
=0

Option A
? :roll:
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Re: If Set A is made up of all 2-digit primes that are one less than a pow  [#permalink]

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New post 13 Apr 2018, 05:11
1
onlynk1 wrote:
Set A ={31,63}

Mean= 47
If we go with proper definition and calculation

SD={-16,+16}
={0/2}
=0

Option A
? :roll:



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.
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Re: If Set A is made up of all 2-digit primes that are one less than a pow  [#permalink]

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New post 16 Apr 2018, 17:07
Bunuel wrote:
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


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
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Re: If Set A is made up of all 2-digit primes that are one less than a pow  [#permalink]

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New post 28 Apr 2018, 17:58
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

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Re: If Set A is made up of all 2-digit primes that are one less than a pow &nbs [#permalink] 28 Apr 2018, 17:58
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