envy5
gmatophobia
Bunuel
If the positive integer N has only three prime factors 2, 3, and 5, what is the value of N ?
(1) N has a total of 12 factors
(2) N > 100
GivenN has three prime factors
Statement 1(1) N has a total of 12 factorsPossible values of N = \(2^2 * 3 * 5\) , \(2 * 3^2 * 5\) or \(2 * 3 * 5^2\)
As we can have multiple values of N, the statement is not sufficient and we can eliminate A and D.
Statement 2(2)N > 100Insufficient, eliminate B.
CombinedThe statements combined we can eliminate the possibility of N = \(2^2 * 3 * 5\) and N = \(2 * 3^2 * 5\) as both of these numbers are less than 100.
Hence N = \(2 * 3 * 5^2\)
Sufficient.
Option CHi would you be able to give an explanation/rule that gives the result of
(1) N has a total of 12 factors - Possible values of N = \(2^2 * 3 * 5\) , \(2 * 3^2 * 5\) or \(2 * 3 * 5^2\)
Finding the Number of Factors of an Integer
First, make the prime factorization of an integer \(n = a^p * b^q * c^r\), where \(a\), \(b\), and \(c\) are prime factors of \(n\), and \(p\), \(q\), and \(r\) are their respective powers.
The number of factors of \(n\) will be expressed by the formula \((p+1)(q+1)(r+1)\). NOTE: this will include 1 and \(n\) itself.
Example: Finding the number of all factors of 450: \(450 = 2^1 * 3^2 * 5^2\)
The total number of factors of 450, including 1 and 450 itself, is \((1+1)(2+1)(2+1) = 2*3*3 = 18\) factors.
According to the above, given that N has only three prime factors 2, 3, and 5, and that N has a total of 12 factors, we'd have \(N = 2^p * 3^q * 5^r\) and \((p + 1)(q + 1)(r + 1) = 12\). Since 12 can be expressed as the product of three positive integers, each greater than 1, in only one way: 12 = 2*2*3, then one from p, q, and r is 2 and the other two are 1.