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# Is the positive integer A divisible by 3?

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Math Expert
Joined: 02 Sep 2009
Posts: 61510
Is the positive integer A divisible by 3?  [#permalink]

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25 Nov 2019, 01:21
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Difficulty:

55% (hard)

Question Stats:

49% (01:41) correct 51% (01:30) wrong based on 40 sessions

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Is the positive integer A divisible by 3?

(1) A + 1 is a prime number
(2) A - 1 is a prime number

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Re: Is the positive integer A divisible by 3?  [#permalink]

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25 Nov 2019, 02:32
E

1---- A could be 4 So no its not divisible by 6, could be 6 so Yes A is divisible by 3.
2---- Same as above examples work
C---- Same as above examples work
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Re: Is the positive integer A divisible by 3?  [#permalink]

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25 Nov 2019, 04:21
#1
A + 1 is a prime number
A= 2,4, we get NO to A divisible by 3
A= 12 , we get Yes to A is divisible by 3
insufficient
#2
A-1 is a prime no
A= 3 , we get yes
A= 4, we get no
from 1 & 2
possible value of A = 4,12
insufficient
IMO E

Bunuel wrote:
Is the positive integer A divisible by 3?

(1) A + 1 is a prime number
(2) A - 1 is a prime number

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VP
Joined: 24 Nov 2016
Posts: 1242
Location: United States
Re: Is the positive integer A divisible by 3?  [#permalink]

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26 Nov 2019, 08:04
Bunuel wrote:
Is the positive integer A divisible by 3?

(1) A + 1 is a prime number
(2) A - 1 is a prime number

primes: 2,3,5,7,11,13,17,19,23…29,30…

(1) A + 1 is a prime number insufic

A+1=31…A=30…30=f(3)
A+1=5…A=4…4≠f(3)

(2) A - 1 is a prime number insufic

A-1=31…A=30…30=f(3)
A-1=5…A=4…4≠f(3)

(1&2) insufic

A=4,30

Ans (E)
Re: Is the positive integer A divisible by 3?   [#permalink] 26 Nov 2019, 08:04
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