If x is a positive integer, is x a prime integer?

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If x is a positive integer, is x a prime integer? [#permalink]

25 Jan 2013, 03:43

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If x is a positive integer, is x a prime integer?

(1) x+1 is a prime number.
(2) x-5 is a prime number.

Can't figure it out.

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Re: If x is a positive integer, is x a prime integer? [#permalink]

25 Jan 2013, 04:47

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If x is a positive integer, is x a prime integer?

(1) x+1 is a prime number. Notice that x cannot be an odd prime, because in this case (odd prime) + 1 = (even number), and the only even prime is 2, but in this case x=1, which is not prime. So, if x=prime, then it must be 2: 2+1=3=prime. Of course, x can be a non-prime, for example 1. Not sufficient.

(2) x-5 is a prime number. If x=7, then the answer is YES but if x=8, then the answer is NO. Not sufficient.

(1)+(2) From (1) we have that if x IS a prime, then it must be 2. But if x=2, them x-5=-3, which is not a prime. Thus x=prime=2 contradicts the second statement. Therefore x cannot be a prime number. Sufficient.

Answer: C.

Hope it's clear.
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Re: If x is a positive integer, is x a prime integer? [#permalink]

20 May 2013, 03:15

suprememodelrus wrote:

If x is a positive integer, is x a prime integer?

(1) x+1 is a prime number.
(2) x-5 is a prime number.

Hi Bunuel, My approach does not look standard. Is it correct?

From statement 1: X= 1,2,4,6,10,12,16,18........
From statement 2: X= 7,8,10,12,16,22........

common from the both statements are 10,12,16..... Thus X is not a prime integer.

Kindly provide algebric approach if possible.

Thanks in advance
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Re: If x is a positive integer, is x a prime integer? [#permalink]

20 May 2013, 09:41

f x is a positive integer, is x a prime integer?

(1) x+1 is a prime number.
(2) x-5 is a prime number.

I think this is a bad question , however ...

from 1

x , x+1 are 2 successive integers ....... if x is prime and since the only 2 prime that are successive are 2,3 therefore if x is prime it is 2 , however x could be 4 as well .... insuff

from 2

The only 2 primes 5 units apart are 7 and 2 , (since all primes > 3 end in 1,3,7,9) if x were to be prime it is 7 , however x could be 8 and (x-5) could be 3 ... insuff

both together

for x to be prime according to the above 2 statements then it has be less than a prime by 1 and larger than another by 5 .... there exist no such a prime

thus sure x is not a prime ( integers if not prime are composite)

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Re: If x is a positive integer, is x a prime integer? [#permalink]

21 May 2013, 01:05

atalpanditgmat wrote:

suprememodelrus wrote:

If x is a positive integer, is x a prime integer?

(1) x+1 is a prime number.
(2) x-5 is a prime number.

Hi Bunuel, My approach does not look standard. Is it correct?

From statement 1: X= 1,2,4,6,10,12,16,18........
From statement 2: X= 7,8,10,12,16,22........

common from the both statements are 0,12,16..... Thus X is not a prime integer.

Kindly provide algebric approach if possible.

Thanks in advance

Yes, that's correct. From both statements x must be even (10, 12, 16, ...), but it cannot be 2. So, x is not a prime number.
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Re: If x is a positive integer, is x a prime integer? [#permalink] 21 May 2013, 01:05

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