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# If p is a positive integer, is 2p + 1 prime? 1. p is prime

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If p is a positive integer, is 2p + 1 prime? 1. p is prime [#permalink]  23 Feb 2008, 07:29
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If p is a positive integer, is 2p + 1 prime?
1. p is prime
2. units digit of p is not prime
Senior Manager
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Re: integers [#permalink]  23 Feb 2008, 08:47
My first thought was E because I wasn't aware of any kind of generalization for prime numbers.

Now taking a deep dive...

1 - insuff. 3 and 7 are primes but give different results
2 - insuff. 4 and 11 give different results.

Combined... 11 and 31 give different results. insuff.

So I am going to stick to E.
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Re: integers [#permalink]  23 Feb 2008, 10:45
marcodonzelli wrote:
If p is a positive integer, is 2p + 1 prime?
1. p is prime
2. units digit of p is not prime

(E)

1) Contradictions for p=3 and p=7. Insufficient
2) Contradictions for p=4 and p=9. Insufficient

1) & 2) Contradictions for p=29 and p=19. Insufficient
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Re: integers [#permalink]  23 Feb 2008, 19:25
E

from stat 1 , p can be 2 and 2p+1 will be prime, or p can be 7 and 2p+1 is not prime. insuff.

from stat 2, p can be 1 and 2p+1 is prime, or, p can be 4 and 2p+1 is not prime. insuff.

together, consider 11 which satisfies both conditions. in this case, 2p+1 is prime. now consider 31, which also satisfies both cases. Here, 2p+1 is not prime. insuff.
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Re: integers [#permalink]  24 Feb 2008, 09:18
I go for E too, OA please ?
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Re: integers [#permalink]  24 Feb 2008, 11:16
marcodonzelli wrote:
If p is a positive integer, is 2p + 1 prime?
1. p is prime
2. units digit of p is not prime

the question is not up to the gmat standard. its statememnts are contradictory to each other. so either the posting is wrong or some information is missing.
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Re: integers [#permalink]  24 Feb 2008, 20:44
GMAT TIGER wrote:
marcodonzelli wrote:
If p is a positive integer, is 2p + 1 prime?
1. p is prime
2. units digit of p is not prime

the question is not up to the gmat standard. its statememnts are contradictory to each other. so either the posting is wrong or some information is missing.

why do you say that?

let's consider 29: it is prime and its unit digit is not prime. the question is from gmatclub challenges
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Re: integers [#permalink]  24 Feb 2008, 23:48
marcodonzelli wrote:
GMAT TIGER wrote:
marcodonzelli wrote:
If p is a positive integer, is 2p + 1 prime?1. p is prime 2. units digit of p is not prime
the question is not up to the gmat standard. its statememnts are contradictory to each other. so either the posting is wrong or some information is missing.
why do you say that?let's consider 29: it is prime and its unit digit is not prime. the question is from gmatclub challenges
lol.....seems i need to be refreshed.
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Re: integers [#permalink]  26 Feb 2008, 08:39
marcodonzelli wrote:
If p is a positive integer, is 2p + 1 prime?
1. p is prime
2. units digit of p is not prime

1: p= prime well p could be 3. Yes 2p+1 is prime or p could be 13. No (27)

2: p could be 11 or 13

Together p can still be 11 or 13

Insuff

E
Re: integers   [#permalink] 26 Feb 2008, 08:39
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# If p is a positive integer, is 2p + 1 prime? 1. p is prime

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