4 A's and 5 B's are to be arranged into a nine letter word. How many different words can be made?
I thought it would be (9!/something) but I don't know how to get the denominator.
We have :
9! / [ (number of repeats of letter A) ! * (number of repeats of letter B) ! ] =
9! / ( 5! * 4!) = 126 identifiable words