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Here is my question:

Consider a multiset $\{n\cdot a, n\cdot b, 1,2,3, \ldots, n+1\}$ of size $3n+1$. Determine the number of $n$-combinations.

I know from my textbook that if you have a $n$-combination multiset of $k$ types, then the answer is $\binom{ n-k-1} {k-1}$ but I am unsure what is $k$ and what is $n$ here. Any help is appreciated.

  • 0
    Why do you think that $n$-combination formula, which I think should be $\binom{n+k-1}{k-1}$, is applicable?2012-09-27

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