How many numbers less than 1000 have an odd number of factors? For example, 6 has 4 factors (1, 2, 3, 6) and 9 has 3 factors (1,3,9).

1 that has an odd number

2=2*1 that has 2 that is no good

In fact, all prime numbers have 2 so none of those match the criterior

4 has 1,2, and 4 That has an odd number

6 is no good either, 1*6,2*3

So at least one of the factors must be a squared number.

Think about that!