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

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Manager
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Joined: 04 Feb 2009
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If p is a positive integer, is 2p + 1 prime? 1. p is prime [#permalink] New post 05 Jul 2010, 01:32
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A
B
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D
E

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46% (04:12) correct 54% (01:06) wrong based on 13 sessions
If p is a positive integer, is 2p + 1 prime?

1. p is prime
2. units digit of p is not prime

may I try here the prime number 2?
Or this option would be inappropriate?
[Reveal] Spoiler: OA
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Re: m14 q 30 [#permalink] New post 05 Jul 2010, 04:39
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Hi,

2 is prime number so you can consider it.

1 - p is prime
p=2, 2p+1=5 is prime
p=3, 2p+1=7 is prime
p=7 2p+1=15 is not prime
So the answer is not sufficient.

2 - The single digit prime numbers are 2,3,5,7
the units digit of p is not prime, hence the units digit of p can be 0,1,4,6,8,9
if p=10, 2p+1=21 is not prime
if p=11, 2p+1=23 is prime
Hence answer 2 is not sufficient.

Now taking 1 and 2 together
prime numbers 2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,53,59,61,67,71,73,79,83,89,97,101,103,107,109,113,127,131,137,139,149,151,157,163,167etc
units digit of p is not prime
possible values of p are:19,29,59,79,89 in other words all prime numbers that are ending with 9.
Assuming p=19, 2p+1=39=3*13 is not prime
Assuming p=29, 2p+1=59 is prime.
Hence taking 1 and 2 together is not sufficient to answer the question.

Ragards,
Jack
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Re: m14 q 30 [#permalink] New post 21 Jul 2010, 23:47
statement 1: counterexample p=7
statement 2: counterexample p=4

statements 1 and 2 cannot go together, so answer is E
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Re: m14 q 30   [#permalink] 21 Jul 2010, 23:47
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