Answer to Question #8838 in Combinatorics | Number Theory for David Harris
how many times is it possible to arrange 8 books on a bookshelf when book 1 and 2 always have to be together but be together anywhere in the order as either 1,2 or 2,1
Let's glue together the first and the second books for a moment. Then, there are 7! possibilities to arrange 7 books on the shelf and 7!*2 = 10080 possibilities to permute these two books.
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