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Why is the number of possible subsequences 2^n?
I'm brushing up on my probability skills and the text asks the following question:
For the sample space {A, B, C, D}, determine how many events are possible.
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?"
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?