Bunuel wrote:
How many prime factors does positive integer n have?
(1) n/7 has only one prime factor.
(2) 3*n^2 has two different prime factors.
Kudos for a correct solution.
VERITAS PREP OFFICIAL SOLUTION:Let’s keep in mind our learning from above while trying to solve this question.
Statement 1: n/7 has only one prime factor.
n/7 has a factor so obviously, it is an integer. Hence n must have a 7 as a factor. So we might jump to the conclusion that n has two prime factors –7 and another one which is left when n is divided off by 7. So n would be something like 7*3 so that n/7 = 7*3/7 = 3 (only one prime factor).
But what we wouldn’t have considered in this case is that n may have multiple 7s so that when a 7 is cancelled in n/7, you would still be left with 7 i.e. if n is 7*7, then n/7 = 7*7/7 = 7. In this case, n has only one distinct prime factor.
So n can have either one or two prime factors. This statement alone is not sufficient.
Statement 2: 3*n^2 has two different prime factors.
This is the same as our previous question. 3n^2 has two different prime factors but n itself can have either one or two prime factors (one of which will be 3). For example, n can be 7 or n can be 3*7. This statement alone is not sufficient.
Using both statements, n could have one or two prime factors i.e. n could be 49 (only one prime factor – 7) or n could be 21 (two prime factors).
Hence, even using both the statements, we cannot say how many prime factors n has.
Answer (E)
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