6c4=15
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Two make combinations you would take 2x1=2 combinations only
42 combinations.
There are 30 different combinations if the order in which the books were chosen counts, so a set of book A and book B is considered different from a set of book B and book A. There are 15 different combinations if it does not matter in which order the books were chosen, so a set of book A and book B is considered the same as a set of book B and book A.
one hundred and twenty
There are sixteen different ways.