How many composite numbers? How many prime numbers? How many even numbers? How many odd numbers?
72 = 2 ^ 3 × 3 ^ 2, so 72 has (3 + 1) * (2 + 1) = 12 factors including 1 and itself. Among these factors, there are 2 prime numbers, that is 2 and 3, so the total number is 12 - 2 - 1 = 9 (total number minus prime number, minus factor 1): 4, 6, 8, 9, 12, 18, 24, 36, 72. Odd number is 1