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My solution: (1) A+1 is a prime number if A=1, A+1 =2 which is prime, but A is not divisble by 3, whereas if A=6, A+1 =7 which is prime, A is divisible by 3 Hence (1) is Not Sifficient

(2) A-1 is prime is A= 6, A-1 = 5 which is prime, A is divisible by 3 but if A=8, A-1=7 which is prime, A is not divisible by 3

(1)+(2) the numbers A-1,A,A+1 are consequtive numbers such that A-1 & A+1 are prime numbers.

Such pairs are 3,4,5----A=4, not divisible by 3 11,12,13-----A=12, divisible by 3 17,18,19-----A=18, divisible by 3

Re: Is A divisible by 3? (1) A+1 is a prime number (2) A-1 is a [#permalink]

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26 Nov 2010, 05:25

nades09 wrote:

Is A divisible by 3?

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

My solution: (1) A+1 is a prime number if A=1, A+1 =2 which is prime, but A is not divisble by 3, whereas if A=6, A+1 =7 which is prime, A is divisible by 3 Hence (1) is Not Sifficient

(2) A-1 is prime is A= 6, A-1 = 5 which is prime, A is divisible by 3 but if A=8, A-1=7 which is prime, A is not divisible by 3

(1)+(2) the numbers A-1,A,A+1 are consequtive numbers such that A-1 & A+1 are prime numbers.

Such pairs are 3,4,5----A=4, not divisible by 3 11,12,13-----A=12, divisible by 3 17,18,19-----A=18, divisible by 3

Hence Answer is E

Is this solution correct?

Thanks NAD

Prime numbers proerty: Exept 2, all the other prime #s are ODD...

qtn: is A=3K, for any integer K.

stmnt 1: A+1 is a prime number

case1-a: A+1 not = 2 but some other prime # then A+1, prime , is an ODD # ==> A is an EVEN that may be divisible by 3. e.g 12

case1-b: A+1= 2, prime # then A = 1 , not divisible by 3 but due to case 1a we can not answer the qtn: hence stmnt 1 NOT SUFF

stmnt 2: A-1 is a prime number

case2-a: A-1 not = 2 but some other prime # then A-1, prime , is an ODD # ==> A is an EVEN that may be divisible by 3. e.g 12

case2-b: A-1= 2, prime # then A = 3 , and divisible by 3 but due to case 2-a we can not answer the qtn: hence stmnt 2 NOT SUFF

1&2 together will eliminate case1-b and case2-b (as both A-1 and A+1 can not be = 2) and retain case1-a and case2-a that say that A is an EVEN but nothing else abt A hence 1&2 together NOT SUFF.

Re: Is A divisible by 3? (1) A+1 is a prime number (2) A-1 is a [#permalink]

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28 Nov 2010, 08:41

1

This post received KUDOS

nades09 wrote:

Is A divisible by 3?

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

My solution: (1) A+1 is a prime number if A=1, A+1 =2 which is prime, but A is not divisble by 3, whereas if A=6, A+1 =7 which is prime, A is divisible by 3 Hence (1) is Not Sifficient

(2) A-1 is prime is A= 6, A-1 = 5 which is prime, A is divisible by 3 but if A=8, A-1=7 which is prime, A is not divisible by 3

(1)+(2) the numbers A-1,A,A+1 are consequtive numbers such that A-1 & A+1 are prime numbers.

Such pairs are 3,4,5----A=4, not divisible by 3 11,12,13-----A=12, divisible by 3 17,18,19-----A=18, divisible by 3

Hence Answer is E

Is this solution correct?

Thanks NAD

Possible values of A using S1: 1,2,4,6,10,12..... NOT SUFFICIENT Possible values of A using S2: 3,4,6,8,12......... NOT SUFFICIENT

Common values in both the lists: 4,6,12...........NOT SUFFICIENT

Re: Is A divisible by 3? (1) A+1 is a prime number (2) A-1 is a [#permalink]

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09 Aug 2017, 08:19

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