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What is the probability that an integer chosen from a set P, which con

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New post 17 Jul 2017, 23:07
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A
B
C
D
E

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  55% (hard)

Question Stats:

63% (01:34) correct 37% (01:41) wrong based on 88 sessions

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New post Updated on: 18 Jul 2017, 02:51
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Bunuel wrote:
What is the probability that an integer chosen from a set P, which contains consecutive positive integers, when divided by 5, leaves a prime remainder?

(1) The value of the range of the integers in set P is 10.
(2) The largest integer in set P is 35.


It should be C

Statement 1: Insufficient

Range gives us the number of sets which is 11. However, the number of prime remainders depends on where the sequence starts.

Say our set P contains 11 integers starting from 1 i.e. set P has elements 1,2,3,4,5,6,7,8,9,10,11. Remainder when divided by 5 would be 1,2,3,4,0,1,2,3,4,0,1 where we have 2,3 (primes) appearing twice.
If we have a set starting with 2 i.e. set P has elements 2,3,4,5,6,7,8,9,10,11,12. Remainder when divided by 5 would be 2,3,4,0,1,2,3,4,0,1,2 so we have 1 extra prime remainder in the form of last 2. This changes probability.

Statement 2: Insufficient
Largest integer is 35, but again applying the same rule, without knowing how many elements are in the range, we cannot be sure of the remainder and its probability of being prime.

Combining, we know the set is 25-35 so we can calculate the total number of primes easily.
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Originally posted by jedit on 18 Jul 2017, 02:18.
Last edited by jedit on 18 Jul 2017, 02:51, edited 2 times in total.
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New post 18 Jul 2017, 02:51
it says that the set has consecutive positive integers and if we pick any set with range of 10 we get 4 prime remainders when divided by 5...must be A..
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Re: What is the probability that an integer chosen from a set P, which con  [#permalink]

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New post 18 Jul 2017, 03:57
oh i have learned my mistake, it must be C
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Re: What is the probability that an integer chosen from a set P, which con   [#permalink] 18 Jul 2017, 03:57
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