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Math Help - Probability with different possibilities

  1. #1
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    Probability with different possibilities

    I completely forget how to do this, any help would be greatly appreciated.

    Alex has a bag containing 5 yellow, 6 blue, and 7 purple marbles. Find the probability of him choosing, without replacement, 2 purples and a blue in any order in 4 picks.

    (^If it were 3 picks, I'd have no problem, but I forget how to do this type of problem...)
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  2. #2
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    Quote Originally Posted by Sconts View Post
    I completely forget how to do this, any help would be greatly appreciated.

    Alex has a bag containing 5 yellow, 6 blue, and 7 purple marbles. Find the probability of him choosing, without replacement, 2 purples and a blue in any order in 4 picks.
    I do not understand what you mean by "picking 4".
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  3. #3
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    Clarification

    Quote Originally Posted by ThePerfectHacker View Post
    I do not understand what you mean by "picking 4".
    Sorry, let me clarify:
    He picks 4 marbles. 2 must be purple, 1 must be blue, and I think the last one can be anything...

    would it be like, 18P4 * (7/18)*(6/17)*(6/16)*(15/15) ?
    or something?
    blah sorry, completely blanking on this section.
    Last edited by Sconts; May 1st 2007 at 06:33 PM. Reason: thought of a new method
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  4. #4
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    Quote Originally Posted by Sconts View Post
    Sorry, let me clarify:
    He picks 4 marbles. 2 must be purple, 1 must be blue, and I think the last one can be anything...
    I recommend to divide the problem into cases.

    Case 1: 2 purble, 1 blue, 1 yellow.

    Case 2: 2 purble, 1 blue , 1 blue = 2 purble , 2 blue.

    Case 3: 2 purble, 1 blue, 1 purble = 3 purble , 1 blue.

    Now, you find the probability for each one, which is much easier. And them you add them up.
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