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Printjmc
counting and probability intermediate
Problem
In how many ways can I arrange 3 different math books and 5 different history books on my bookshelf, if I require there to be a math book on both ends?
Solution
Let's deal with the restriction first.
The restriction is that we have to place a math book on both ends. We have 3 choices for the math book to place on the left end, and then 2 choices for the math book to place on the right end.
Then we simply need to arrange the other 6 books in the middle. This is a basic permutation problem, so there are ways to arrange the 6 remaining books.
So there are a total of ways to arrange the books on the bookshelf.
The restriction is that we have to place a math book on both ends. We have 3 choices for the math book to place on the left end, and then 2 choices for the math book to place on the right end.
Then we simply need to arrange the other 6 books in the middle. This is a basic permutation problem, so there are ways to arrange the 6 remaining books.
So there are a total of ways to arrange the books on the bookshelf.
Final answer
4,\!320