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Is n prime?

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by Brent@GMATPrepNow » Thu Jan 15, 2009 4:29 pm
If n is an integer greater than 1, is n prime?
(1) (n-1)! is not divisible by n
(2) n = (2^k) – 1, where k is a positive integer greater than 1
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Source: — Data Sufficiency |
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Re: Is n prime?

by logitech » Thu Jan 15, 2009 5:13 pm
If n is an integer greater than 1, is n prime?

(1) (n-1)! is not divisible by n

n=3
2! = 2 is not divisible with N ; 3 is prime

n=4
3! = 6 is not divisible with N ; 4 is not prime

INSUF

(2) n = (2^k) – 1, where k is a positive integer greater than 1[/quote]

n = 3 PRIME ( k=2)
n= 15 NOT prime ( k =4)

Together

n=3 --- PRIME (n-1)! not divisible
n=7 ---- PRIME (n-1)! not divisible
n= 31 ----PRIME (n-1)! not divisible

SUFFICIENT because n-1 will eliminate the PRIME number from the factors.

Hence C
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by dmateer25 » Thu Jan 15, 2009 5:16 pm
(2) n = (2^k) – 1, where k is a positive integer greater than 1

This tells us that n is odd but no necessarily prime.

for example:
when k = 4, n will equal 15, which is not prime.
when k = 3, n will equal 7, which is prime.

INSUFF

(1) (n-1)! is not divisible by n

when n =4, (n-1)!=6, which is not divisible by 4 and n is not prime.

when n = 5 (n-1)!=24, which is not divisible by 5 and n is prime.

Insuff



Combined:

From stmt 2 we know n must be odd. and from stmt 1 in order for n to be odd it must be prime.

I will go with C
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by Brent@GMATPrepNow » Thu Jan 15, 2009 5:16 pm
Nice work, Logitech. The answer is C.
Now it's you who is ruining my night :D
Hw many more days before the big test?
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