Question

How many positive integers less than or equal to 500 have exactly 3 positive divisors?

Answer 1

all numbers automatically have 2 divisors, 1 and themselves

the ONLY way it can have EXACTALLY one more divisor is if
1) that divisor is prime
2) that divisor is the square root of the number

basically, you are looking at all the prime numbers from [1, sqrt(500)), then square them

Answer 2

sqrt(500)= 22.36

Answer 3

2,3,5,7,11,17,19

Answer 4

squared

Answer 5

4,9,25,121,289,361