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From stmt 1, N is a multiple of 5 in such that N is obtained by multiplying 5 with another prime number.

Thus, N can be 10, 15, 25, 35, 55...... i.e. N can have 1 prime factor (e.g. 25) or 2 (e.g. 10, 15,...)....hence not sufficient.

From stmt2, 3 is already a prime factor. Hence, in order for 3N^2 to have two different prime factor, N^2 has to be a perfect square of a prime number. This implies that N itself is a prime number and hence will have only one prime factor. Hence, sufficient.

How many prime factors does positive integer N have? (1) N/5 is a prime number. (2) 3N^2 has two different prime number.

D.

If N/5 is a prime number, then N=5*x where x is a prime number. So N has only two prime factors.

if 3*N^2 has two different prime factors, then 3 is one of them and N is the other. In this case, N has to be a prime number and hence will have only one prime factor

1) N = 5a, where a is any prime number. N can have one or two prime factors. Not sufficient 2) 3N^2 has two different prime factors. Not sufficient, because N can have one prime factor, or two prime factors (f.ex. N=21=3*7).

Combine both - N can be 25 or 15, i.e. two prime factors.
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How many prime factors does positive integer N have? (1) N/5 is a prime number. (2) 3N^2 has two different prime number.

(1) N/5 is a prime number. N= 5* x it doesn't matter.. whether x=5 or 3 .. N has two prime factors. N=5*5 ( two prime factors.... though not two different prime factors.)

Question is How many prime factors does positive integer N have? Ans 2

(2) 3N^2 has two different prime number.[/ N must be prime number. sufficient.

D.

If question Says How many different prime factors does positive integer N have? then Ans would be C.
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Your attitude determines your altitude Smiling wins more friends than frowning

How many prime factors does positive integer N have? (1) N/5 is a prime number. (2) 3N^2 has two different prime number.

(1) N/5 is a prime number. N= 5* x it doesn't matter.. whether x=5 or 3 .. N has two prime factors. N=5*5 ( two prime factors.... though not two different prime factors.)

Question is How many prime factors does positive integer N have? Ans 2

(2) 3N^2 has two different prime number.[/ N must be prime number. sufficient.

D.

If question Says How many different prime factors does positive integer N have? then Ans would be C.

another good catch gosh... hopefully the real test will leave no room for ambiguity .

1) N = 5a, where a is any prime number. N can have one or two prime factors. Not sufficient 2) 3N^2 has two different prime factors. Not sufficient, because N can have one prime factor, or two prime factors (f.ex. N=21=3*7).

Combine both - N can be 25 or 15, i.e. two prime factors.

Guys, I messed up completely. If N can be 25 or 15, then answer is E - assuming we are looking for different prime factors. If we are looking for number of prime factors, not necessarily different, then it should be A. OA?
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If I take the number 25, do I say, it has two prime factors? does it not have only one prime factor and that is 5? 25 can be divided by 5 and 5 is a prime factor. why do I have to count 5 again?

gmatclubot

Re: prime factors of n
[#permalink]
05 Sep 2008, 00:26