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How would you show that $$\binom{n}{k}\binom{k}{m}\binom{m}{r} = \binom{n}{r}\binom{n-r}{n-m}\binom{n-m}{k-m}$$ for $n\geq k\geq m\geq r$ ?

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    Use the definition of the Binomial coefficient.2016-02-09

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