Re: Probability problem - couting number of distinct elements

> Hi all, i have the following problem: suppose we have
> a bag with N
> balls with different colors. Randomly, i take one
> ball from the bag,
> record the color, and put it back in the bag. Now,
> the question is:
> after taking k balls from the bag, what's the
> expected number of
> different colors i have seen? Any suggestion? Thanks!
> --Ricardo
>

What's the probability that the red ball has been chosen?
Think of that as the expected value of a variable which
is 1 when the ball is chosen, 0 when not.
Then use linearity of expectation:
expected value of sum of two random variables is
the sum of their individual expected values.
.

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