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Probability of balls chosen from an urn

  1. Sep 25, 2007 #1
    1. The problem statement, all variables and given/known data
    An urn contains 3 white and 4 red balls. A ball is chosen at random ( in a frequency sense) and replaced. What is the probability of the first 5 balls chosen exactly 2 will be white.


    2. Relevant equations



    3. The attempt at a solution

    Really I haven't made much progress. I do not understand the significance of the balls being replaced. If they weren't replaced that would make sense to me. But with them being replaced I have no idea. Thanks for any help and advice.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Sep 25, 2007 #2

    nicksauce

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    Because they are being replaced, the chance of getting a white ball will always be 3/7.

    So it's like, what is the probability that if you flip a coin 5 times you will get exactly two heads (only instead of 50/50, then chance of getting heads in 3/7).
     
  4. Sep 25, 2007 #3

    Dick

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    The significance of the balls being replaced just means every time you draw a ball you have a 3/7 chance of it's being white and 4/7 of it's being red. So what are the odds of getting 2 white and 3 red. Binomial distribution. Go for it Nietzsche!
     
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