What numbers are proper factors?

Answer 1

That means that one number is a factor of the other number, but that it's not the number itself.

For example: the factors of 9 are 1, 3, and 9.

The proper factors of 9 are 1 and 3.

Answer 2

A proper factor of a natural number n, is any number p such that 1 < p < n and p divides n (without remainder). Clearly, n must be a composite number for it to have a proper factor.