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An urn contains four white balls and six black. Three balls are drawn with replacement. Let $x$ be the number of white balls. Calcaulate $E (x)$, $VAR(x)$ and $\sigma x$.

I don't know how to calculate $E(x) =\sum\limits_{i=1}^{n} X_i P(X_i)$.

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    Hint: Number and name the balls as $W_1, W_2, W_3, W_4$ and $B_1, B_2, \ldots, B_6$, and make a list of all possible outcomes of three draws with replacement. Then figure out the corresponding value of $X$ for each outcome to deduce the probability mass function of $X$ and proceed. (There are other ways of doing this problem if you know about the binomial distribution that I won't get into).2012-05-25
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    I Know that take a white ball is: $\frac{4}{10}$ and black is: $\frac{6}{10}$.2012-05-25

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