## permutation or combination

1. The problem statement, all variables and given/known data

in how many different ways can 7 different books be arranged in a row if
a. 3 specified books must be together,
b. two specified boks mus occupy both ends

2. Relevant equations

i dont udnerstand wether it is a permutation or a combination.

3. The attempt at a solution

a. 4! x 7!
b. 7 x 5! x 6
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 no. Try first this: how many permutations (that means unique orderings) of the 7 books can you have if we have 7 distinct books? If we tie three books together, how is that in comparision to a single book? In other words, can we treat it like a single book or not? Then how many books or book groups do we have to recombine? If we have two specified books on either end, how many books can we move around?

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