If p is a positive integer, is 2p + 1 prime? 1. p is prime : Quant Question Archive [LOCKED]
Check GMAT Club Decision Tracker for the Latest School Decision Releases http://gmatclub.com/AppTrack

 It is currently 21 Jan 2017, 17:11

### GMAT Club Daily Prep

#### Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

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

Author Message
VP
Joined: 22 Nov 2007
Posts: 1092
Followers: 8

Kudos [?]: 488 [0], given: 0

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

### Show Tags

23 Feb 2008, 07:29
1
This post was
BOOKMARKED
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

If p is a positive integer, is 2p + 1 prime?
1. p is prime
2. units digit of p is not prime
Senior Manager
Joined: 15 Aug 2007
Posts: 252
Schools: Chicago Booth
Followers: 1

Kudos [?]: 69 [0], given: 0

### Show Tags

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.
Senior Manager
Joined: 26 Jan 2008
Posts: 267
Followers: 4

Kudos [?]: 101 [0], given: 16

### Show Tags

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
_________________
SVP
Joined: 28 Dec 2005
Posts: 1575
Followers: 3

Kudos [?]: 147 [0], given: 2

### Show Tags

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.
Manager
Joined: 20 Sep 2007
Posts: 106
Followers: 1

Kudos [?]: 69 [0], given: 0

### Show Tags

24 Feb 2008, 09:18
I go for E too, OA please ?
SVP
Joined: 29 Aug 2007
Posts: 2492
Followers: 68

Kudos [?]: 735 [0], given: 19

### Show Tags

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.
_________________

Gmat: http://gmatclub.com/forum/everything-you-need-to-prepare-for-the-gmat-revised-77983.html

GT

VP
Joined: 22 Nov 2007
Posts: 1092
Followers: 8

Kudos [?]: 488 [0], given: 0

### Show Tags

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
SVP
Joined: 29 Aug 2007
Posts: 2492
Followers: 68

Kudos [?]: 735 [0], given: 19

### Show Tags

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.
_________________

Gmat: http://gmatclub.com/forum/everything-you-need-to-prepare-for-the-gmat-revised-77983.html

GT

CEO
Joined: 29 Mar 2007
Posts: 2583
Followers: 19

Kudos [?]: 421 [0], given: 0

### Show Tags

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
Display posts from previous: Sort by