There are X
kinds of sticker. Once the player buys a bag he can get Y
different stickers. (expectantly) How many bags does he have to buy so that he can collect all kinds of sticker? How about the standard deviation?
expect value of trying times
0
$\begingroup$
probability
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1What did you try? – 2012-07-23
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3Please see the classic [coupon collector problem](http://en.wikipedia.org/wiki/Coupon_collector%27s_problem) wiki page, corresponding to $Y=1$. – 2012-07-23