P\left( {X = k} \right) = \frac{{\left( \begin{array}{c}
 r \\ 
 k \\ 
 \end{array} \right)\left( \begin{array}{c}
 m - r \\ 
 n - k \\ 
 \end{array} \right)}}{{\left( \begin{array}{c}
 m \\ 
 n \\ 
 \end{array} \right)}}