+422 I have 5 different biology textbooks and 4 different economics textbooks. In how many ways can I place the 9 textbooks on a bookshelf, in a row, if there must be an economics textbook in the middle, and there must be a biology textbook at each end?
There are 4*P(5,2) = 240 ways to place an economics text book in the middle as well as place a biology textbook at each end. There are 5! = 120 ways to arrange the remaining textbooks.
So the answer is 240*120 = 28800.
+118667 I have 5 different biology textbooks and 4 different economics textbooks. In how many ways can I place the 9 textbooks on a bookshelf, in a row, if there must be an economics textbook in the middle, and there must be a biology textbook at each end?
B1 B2 B3 B4 B5 E1 E2 E3 E4
B***E***B
5 * 4 * 4 * 6! = 57600
