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Math Help - Smallest number of items from given probability

  1. #1
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    Smallest number of items from given probability

    Hey All,

    Another problem I'm confused with.

    A bag contains several blue discs and several yellow discs. Find the smallest number of discs which should be taken out of the bag, if the probability that at least 3 of those removed are of the same color is 1.

    Only thing I can see is that least number must be > 3. How do I proceed if the no of items is unknown. I have feeling I am missing something simple, a hint would be great.

    Thanks again for all your help!
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  2. #2
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    its hard to give a useful hint that isn't the answer itself. Try thinking along these lines:

    Suppose i took out 2 discs, it it possible that no 3 of them are the same color?
    Yes, (obviously since there aren't 3 discs!). eg 2 could be blue, none yellow.

    Suppose i took out 3 discs, it it possible that no 3 of them are the same color?
    Yes, eg 2 could be blue, one yellow (or the other way around)

    Keep going until you find that it is impossible (it wont take long!)
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  3. #3
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    5! either way will have 3 of same color. Thanks, seems so simple now that you showed the way.
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