6! = 1 x 2 x 3 x 4 x 5 x 6.
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There are 2*5! = 240 ways.
The number of ways that 15 books can be arranged on a shelf is the same as the number of permutations of 15 things taken 15 at a time. This is 15 factorial, or 15!, and is 1,307,674,368,000.
5040
5
7 factorial, or 5,040.