From set theory, the factorial combinations are 2^N where N are the number of induvidual mambers of the set and that includes the null set. If we require that one element be the minimum to form a set, this eliminates the empty set (and there is only one empty set) so we must subtract one from the formula. The correct answer is therefore 2^N-1. In this case that is 2^5–1 = 31.
