SOLUTION: There are 21 identical Physics books and 19 identical Chemistry books. In how many ways they can be placed in a row, keeping no two Chemistry books together.

Question 892111: There are 21 identical Physics books and 19 identical Chemistry books. In how many ways they can be placed in a row, keeping no two Chemistry books together.
Answer by robertb(5830) (Show Source): You can put this solution on YOUR website!
Line up the 21 Physics books in a row. Then there are 22 places where the Chemistry books can be placed. Hence the total number of ways the Chem and physics books can be placed in a row with no two Chem books together is

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