Question

How many three digits number have exactly three factor

Answer 1

For a number, N, to have exactly three factors, its prime factorization must be of the type p^2, where p is a prime number. In other words, for a 3-digit number to have exactly three factors then it must be square of a prime number.

p=11, 13, 17, 19, 23, 29 and 31

So, the numbers are N =p^2 = 121, 169, 289, 529, 841 and 961