2, 4, 5, 10
Yes. 10 is a proper factor of 20, among others.
a proper factor is a factor.
a^2-a-20
No. Prime numbers don't have proper factors.
It can be. 18 is a proper factor of 36.
It can be. 25 is a proper factor of 50.
It can be. 24 is a proper factor of 48.
It can be. 42 is a proper factor of 84.
It depends on the number being factored. 22 is a proper factor of 44. 22 is a factor, but not a proper factor, of 22.
It can be. 22 is a proper factor of 44.
It can be. For example, 36 is a proper factor of 72 and of 108.
The only proper factor of four is two.