Math Destroyer Help

[punchdrunk7]

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So can anyone explain to me how to do problem 40 in Test #1.

#40 In how many ways can 4 of the same (identical) physics books, 3 of the same algebra books, and 2 of the same chemistry books be arranged on a shelf?
A-1260
B-1628
C-362,880
D-37,756
E-40,137

I looked at the solution but I'm still lost.

 

[silveryhair]

First count up the number of books, then the equation is:

Total number of books!/(Number of repeated books!)

Thus:

9!/4!3!2!