I'm brushing up on my probability skills and the text asks the following question:

I understand that an event is a subset of a sample space, so the question is essentially asking "how many subsets of {A, B, C, D} are there?"For the sample space {A, B, C, D}, determine how many events are possible.

Other than the brute force method of listing all the different subsets I can't figure out how to crack this question.

The book's answer is 16.

Help?

mb