The number 15 in binary is represented as 1111. Binary is a base-2 numeral system, using only digits 0 and 1. Each binary digit represents an increasing power of 2, from right to left. For 15, this means 1*2^3 (8) + 1*2^2 (4) + 1*2^1 (2) + 1*2^0 (1), which sums to 15. Therefore, 1111 in binary equals 15 in decimal.