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Prime Number

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Re: Prime Number

by Stuart@KaplanGMAT » Fri Jul 18, 2008 8:59 pm
smclean23 wrote:Is the prime number p equal to 37?
(1) p = n2 +1, where n is an integer.
(2) p2 is greater than 200.
(1) if n = 6, then p = 37, so the answer could be "yes".

However, if n=2, then p=5 (another prime), so the answer could be "no".

Could be yes, could be no: insufficient.

(2) 37^2 is greater than 200, so the answer could be "yes"

17^2 is greater than 200, so the answer could be "no".

Could be yes, could be no, insufficient.

Together:

we saw that it's possible to get a "yes" answer for both statements, so now we need to see if we can get a "no" answer.

p=5 is no longer allowed, since 5^2 is NOT greater than 200.

However, p=17 IS still allowed, since 17 = 4^2 + 1. So, p could also equal 17.

At the end of the day, we can still get both a "yes" and a "no" answer: choose (E).
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by singalong » Tue Sep 06, 2011 4:33 am
Stuart Kovinsky wrote:
At the end of the day, we can still get both a "yes" and a "no" answer: choose (E).
Hi Stuart.I am finding it tough to understand this logic in DS. Could you explain more about this approach in GMAT or perhaps guide me to an existing explanation?
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