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Homework Help: Selecting from an Urn. Conditional probability

  1. Mar 17, 2012 #1
    1. The problem statement, all variables and given/known data

    A box contains 4 red balls and 6 white balls. A sample size of 3 is drawn without replacement from the box. What is the probability of obtaining 1 red ball and 2 white balls given that at least 2 of the balls in the sample are white?

    2. Relevant equations



    3. The attempt at a solution
    Well if I got at least two white balls that means that I either have 2W or 3W. If I have 2W, then I can chose among the remaining 4W and 4R. Hence the probability is 1/2 given that i already otained 2W. If i have 3W, the probabilty is 0 given that i already obtained 2W. Hence my answer is 1/2. BUT... im wrong. the answer is 3/4. Im not sure where i went wrong here.
     
  2. jcsd
  3. Mar 17, 2012 #2

    tiny-tim

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    Science Advisor
    Homework Helper

    hi torquerotates! :smile:
    no, that's if the question says "given that the first 2 of the balls in the sample are white"

    (or any particular pair)

    find P(W ≥ 2) and P(W = 2) :wink:
     
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