Jano Chmura
Jano Chmura voted up Charles Davis' answer

Computer math is something of a different breed. You use binary and hexadecimal math. Binary uses only 0, and 1. Hex is 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F. In Binary 15 is equal to 1111, in Hex 15 is equal to F.