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What's wrong with my logic on this combinatorial/probability question?

  1. Aug 20, 2012 #1
    1. The problem statement, all variables and given/known data


    3. The attempt at a solution

    If at least 3 balls must be blue, then the possible ways of selecting at least 3 blues are:

    r, b, b, b
    b, r, b, b
    b, b, r, b
    b, b, b, r
    b, b, b, b

    where r = red ball, and b = blue ball

    So there are 5 ways to draw 3+ blue.
  2. jcsd
  3. Aug 20, 2012 #2


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    Science Advisor
    Homework Helper

    The question didn't really spell out what 'different' means. I think they want you to treat the balls as distinguishable and probably are not concerned with the order.
  4. Aug 21, 2012 #3
    The problem with your logic is that there are 9 different blue balls and you don't account for all of them being possibly drawn.

    My hint for you is to consider combinations, i.e. how many ways are there to choose 3 blue balls out of 9?
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