2005 AIME I Problems/Problem 3
Problem
How many positive integers have exactly three proper divisors, each of which is less than 50?
Solution
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Having three proper divisors means that there are 4 regular divisors. So the number can be written as 
where 
and 
are primes. The primes under fifty are 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, and 47. There are 15 of them. So there are 
such numbers.
