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jmc

counting and probability intermediate

Problem

Suppose that I have different books, of which are math books. In how many ways can I stack my books on a shelf if I do not want the math books to be next to each other?
Solution
We first place the non-math books. There are choices for the first book, choices for the second book, choices for the third book, and choice for the last book. Then we have to put the two math books between the four non-math books such that there is at least one non-math book between the two math books. We see there is a total of openings created by the four non-math books. So the first math book has choices, and the second math book has choices.

So the total number of ways the books can be placed is
Final answer
480