It is 2^n because for each job it is either take it or not take it (0,1). In other words, there are two possibilities for each job. For n jobs it is 2^n.
n! Is total of permutations for n jobs, it is used when you have taken n jobs but you want to know all the possible sequences.
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solved Why 2^n and not n!: The Algorithm Design Manual: 1.2 Selecting the Right Job (page 10) [closed]