Hard
Between 1 and 100, how many numbers are there with exactly three factors?
Explanation
Explanation
Numbers with exactly three factors are squares of prime numbers (p squared), since their divisors are 1, p, and p squared. The prime squares under 100 are 4, 9, 25, and 49 (from primes 2,3,5,7), giving 4 such numbers.
PRO TIP
Re-check your working by substituting the answer back into the original question.