Question

find the number between 1 and 100 exactly have three factors

Answer 1

As it turns out, the only positive integers with exactly three factors are the squares of primes. For instance, the factors of 9 are 1, 3, and 9, and the factors of 49 are 1, 7, and 49. If you find all of the factors of a non-square, you can "pair off" the factors.