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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?

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    What did you try?2012-07-23
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    Please see the classic [coupon collector problem](http://en.wikipedia.org/wiki/Coupon_collector%27s_problem) wiki page, corresponding to $Y=1$.2012-07-23

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