Suppose that I have 6 different books, 2 of which are math books. In how many ways can I stack my 6 books on a shelf if I do not want the math books to be at the ends?
+9676 You can choose two non-math books out and place them at the ends first, to ensure math books never will be at the ends.
Number of ways to choose two non-math books = .
But you have two ways to choose which book to place at the bottom, so you need to multiply 2.
Then, the remaining 4 books can be rearranged in any order in the middle, so multiply by another , you get the total number of ways as .
