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Math Help - expection

  1. #1
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    expection

    an urn contains K balls, of which B are black and K-B are white. fred drasw a ball at random and replaces it, N times. What is the expection of the number of times a black ball (denote as n_b) is drawn?

    the answe is expection = N (B/K) but i cant get it.

    what i did was expection= sum of all possible n_b X probability distribution of n_b

    which in this case is a binomial distribution.
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  2. #2
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    Re: expection

    Quote Originally Posted by alexandrabel90 View Post
    an urn contains K balls, of which B are black and K-B are white. fred drasw a ball at random and replaces it, N times. What is the expection of the number of times a black ball (denote as n_b) is drawn?

    the answe is expection = N (B/K) but i cant get it.
    Surely you have done the expected value of any binomial distribution?
    Look that up if you have not.
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