I think it's (M*N)! / (N!)**M.

There are (M*N)! ways to organize all the items; there are N! identical arrangements of item #1-of-M, so you divide by that; there are N! if the ith item of M, so you also have to divide by that... in the end, there are N groups of M! you need to divide by.

In reply to Re: Combinatorics formula by pryrt
in thread Combinatorics formula by BrowserUk

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