2005 AIME I Problems/Problem 3
Problem
How many positive integers have exactly three proper divisors, each of which is less than 50?
Solution
or 
where p and q are distinct primes. In the first case, the three proper divisors of n are 1, p, and q. Because there are 15 prime numbers less than 50, there are 
numbers of the first type.There are four integers of the second type because 2, 3, 5, and 7 are the only primes with squares less than 50. Thus there are 
integers that meet the given conditions.
