# What's the expectation of the number of draws until I get a black ball from a urn contains black and white balls? [duplicate]

Imagine we have a urn in which there are $b$ balls, of which $a$ are black and $b-a$ are white. Now, we keep drawing balls (without putting back) until we get a black ball, let $X$ be the number of draws. What's the expectation of $X$? And does $X$ has some specific distribution?

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–  pedrosorio Nov 4 '12 at 18:42
–  user17762 Nov 4 '12 at 18:44