How many two digit numbers have exactly 4 positive factors? (Here 1 and the number n are also considered as factors of n .)

Step-by-step solution

Two digit numbers with 4 positive factors are either the product of two prime numbers or perfect cubes. i.e. 2 5,2 7 , _ _ _ _ 2.47 3.5, 3.7, 3.11, _ _ _ _ 3.31 5.7, 5.11, 5.13, _ _ _ _ 5.19 7.11, 7.13; and 3^3 Total numbers =30