I need help!

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olylo: Newbie | Next Rank: 10 Posts; Posts: 7; Joined: Wed Mar 10, 2010 5:00 pm

by olylo » Fri Sep 30, 2011 10:43 pm

Hi everyone,

I need help please to understand how can I solve this DS within 2 min
The question is :

is the number M a prime number?

1) M+7 is a prime number.

2) M-7 is a prime number.

Thanks

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Source: Beat The GMAT — Data Sufficiency | Fri Sep 30, 2011 10:43 pm

GmatMathPro: GMAT Instructor; Posts: 349; Joined: Wed Sep 28, 2011 3:38 pmFollowed by: 54 members; Location: Austin, TX; Thanked: 236 times; GMAT Score:770

by GmatMathPro » Fri Sep 30, 2011 11:36 pm

A useful thing to keep in mind on problems like these is that every prime number besides 2 is odd.

Statement 1: If M is prime, it is either 2 or some odd prime. It can't be 2 because M+7 would then be 9, which is not prime. If M is an odd prime, then M+7 is odd+odd which is always even and greater than 2, and would not be prime. Therefore, M cannot be prime if M+7 is prime. SUFFICIENT

Statement 2: Similar reasoning. M can't be 2 again, because that would make M-7=-5, and M can't be an odd prime because that would make M-7 an even number that is not 2. Therefore, M cannot be prime if M-7 is prime.

So the answer is D

Pete Ackley
GMAT Math Pro
Free Online Tutoring Trial

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olylo: Newbie | Next Rank: 10 Posts; Posts: 7; Joined: Wed Mar 10, 2010 5:00 pm

by olylo » Sat Oct 01, 2011 12:28 pm

Thank you so much for solving the question.
Your explanation is very clear.

Thanks again

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