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 Your Progress

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

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

It appears that you are browsing the GMAT Club forum unregistered!

Signing up is free, quick, and confidential.
Join other 500,000 members and get the full benefits of GMAT Club

Registration gives you:

Tests

Take 11 tests and quizzes from GMAT Club and leading GMAT prep companies such as Manhattan GMAT,
Knewton, and others. All are free for GMAT Club members.

Applicant Stats

View detailed applicant stats such as GPA, GMAT score, work experience, location, application
status, and more

Books/Downloads

Download thousands of study notes,
question collections, GMAT Club’s
Grammar and Math books.
All are free!

Thank you for using the timer!
We noticed you are actually not timing your practice. Click the START button first next time you use the timer.
There are many benefits to timing your practice, including:

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

Show Tags

05 Jul 2010, 05:39

2

This post received KUDOS

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.

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

Show Tags

23 Aug 2016, 23:55

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________

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

Show Tags

10 Jan 2017, 22:38

Statement-1: Consider p=2, 3, 5, 7, 11. In that case, values of 2p+1 are 5, 7, 11, 15 and 23. Here, 15 is not Prime. Hence we cannot be sure if 2p+1 is prime or not. So, statement 1 alone is not sufficient to answer the question.

Statement-2: Consider p=14, 16, 18. In this case, values of 2p+1 are 29, 33, 37. Here, 33 is not prime. Hence, we cannot be sure if 2p+1 is prime or not. So, statement 2 alone is not sufficient to answer the question.

Both statements together: Here, select p=19, 29, 59. Here, the values of 2p+1 are 39, 59, 109. Here, 39 is not prime. Still inconclusive.

Best Schools for Young MBA Applicants Deciding when to start applying to business school can be a challenge. Salary increases dramatically after an MBA, but schools tend to prefer...

Marty Cagan is founding partner of the Silicon Valley Product Group, a consulting firm that helps companies with their product strategy. Prior to that he held product roles at...