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Math Help - probability of picking a ball (this is easy :P)

  1. #1
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    probability of picking a ball (this is easy :P)

    if there are n balls in a bag, 1 black and n-1 white, and i take k balls out of the bag, how come the probability of taking a black is k/n?

    I get that it works if you try adding the probabilities of getting black on the 1st, 2nd..kth attempt, but whats the logic behind it?
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  2. #2
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    Just do the caculation: \displaystyle{ \frac{\binom{1}{1}\binom{n-1}{k-1}}{\binom{n}{k}}}=?
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  3. #3
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    thanks, why is that the calculation?
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  4. #4
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    Quote Originally Posted by Aquafina View Post
    thanks, why is that the calculation?
    Well you choose one black ball from 1 and k-1 white balls from n-1 white balls.
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