Bunuel
If x is a positive integer, what is the number of different positive factors of 24x ?
(1) x is a two-digit number
(2) x^2 has 3 positive factors
Are You Up For the Challenge: 700 Level Questions Official Solution:If \(x\) is a positive integer, what is the number of different positive factors of \(24x\) ? Factorize: \(24x=2^3*3*x\)
(1) \(x\) is a two-digit number
Clearly insufficient.
(2) \(x^2\) has 3 positive factors
The above means that \(x\) is a prime number. Only the squares of primes (\(p^2\)) have three factors: 1, \(p\), and \(p^2\)
If \(x=11\) (
or any other prime except 2 and 3), then \(24x=2^3*3*11\) will have \((3+1)(1+1)(1+1)=16\) factors;
If \(x=2\), then \(24x=2^4*3\) will have \((4+1)(1+1)=10\) factors.
If \(x=3\), then \(24x=2^3*3^2\) will have \((3+1)(2+1)=12\) factors.
Not sufficient.
(1)+(2) Since (1) says that \(x\) is a two-digit number, then \(x\) cannot be 2 or 3, so it must be a two-digit prime. For any two digit prime, \(24x=2^3*3*x\) will have \((3+1)(1+1)(1+1)=16\) factors. Sufficient.
Answer: C