# Math Help - Help:calculate the expectation

1. ## Help:calculate the expectation

k obeys hypergeometric distribution. Every time, I pick up k balls from the population N balls, which have m red balls. I want only one red ball in each pickup event.
Here is my question, how can I calculate the expectation of k.

$E(k)=\sum_{k=1}^{N-m}\frac{(^m_1)(^{N-m}_{k-1})}{(^N_k)}k=?$

very thank for your help.

2. If you rewrite the coeficients as factorials, it may become more obvious.

Maybe not. Try exploring a little.