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Lab#2 DS Yes-No Testing #107. Dont understand.

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spenceryuan
 Post subject: Lab#2 DS Yes-No Testing #107. Dont understand.
 Post Posted: Mon Apr 25, 2011 9:12 pm 
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Course Students


Posts: 3
The question states.

107. Can the positive integer n be written as the sum of two different prime numbers?

(1) N is greater than 3

(2) N is odd.

In the example it says the answer is E. What I don't understand is why isn't the first piece of information sufficient to solve the problem?

If you get the number 5, which satisfies the criteria, why wouldn't that be enough to answer the question. The answer to the question would be YES, the number 5 can be written as the sum of 2 and 3. Not understanding why the answer wouldn't stop there once you get an answer yes.
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smohit04
 Post subject: Re: Lab#2 DS Yes-No Testing #107. Dont understand.
 Post Posted: Mon Apr 25, 2011 10:55 pm 
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Course Students


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Because by either of the option we are not getting the unique value of integer N.

If we say N is greater than 3, then N could be 11 also, which is not the sum of two prime numbers. So we cannot say that option 1 is sufficient.

N is odd: N could be 5 (sum of two primes) or 11 (not the sum of two primes) so option two is also not sufficient.

Combining both also won't give any unique value to answer the question.
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jnelson0612
 Post subject: Re: Lab#2 DS Yes-No Testing #107. Dont understand.
 Post Posted: Sat Apr 30, 2011 1:09 pm 
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ManhattanGMAT Staff


Posts: 1857
Thank you smohit. Nice examples! Note, testing numbers is a smart way to tackle one like this. Our goal is always to try to find two separate values and prove insufficiency.

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Jamie Nelson
ManhattanGMAT Instructor
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