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Math Help - Bin with balls

  1. #1
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    Bin with balls

    Not sure if I did part a correctly. Part B, not even sure how to do it.
    Problem:
    A bin contain balls numbered 1 through 5. Balls are removed, one at a time, until a ball with a similar value tan the previous number is drawn.

    a. Find the probability that you stop after three draws (the third ball has a smaller value)

    I first figured that there were 14 possible formations that would meet the requirements. They are:
    (2,3,1), (2,4,1), (2,4,3), (2,5,1), (2,5,3), (2,5,4), (3,4,1), (3,4,2), (3,5,1), (3,5,2), (3,5,4), (4,5,1), (4,5,2), (4,5,3)

    I then figured out the total possible formations 5P3 = 60

    14/60 = .2333 is the probability that I would stop after three draws. Was I correct in completing this problem?

    b. Let X represent the value of the ball that is the first ball with a smaller value. Determine the probability mass function on X.
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  2. #2
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    There are 6 more draws that would satisfy the conditions for part A:

    (1, 3, 2), (1, 4, 2), (1, 4, 3), (1, 5, 2), (1, 5, 3), (1, 5, 4)
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  3. #3
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    Quote Originally Posted by cag31 View Post
    Part B, not even sure how to do it.
    Problem:
    A bin contain balls numbered 1 through 5. Balls are removed, one at a time, until a ball with a smaller value to the previous number is drawn.


    b. Let X represent the value of the ball that is the first ball with a smaller value. Determine the probability mass function on X.
    Here's a hint to help with part B...

    The ways that 1 can be the first smaller number are

    2,1
    2,3,1......2,4,1.....2,5,1
    2,3,4,1...2,3,5,1...2,3,4,5,1...2,3,5,4,1
    2,4,3,1
    ...2,4,5,1...2,4,3,5,1...2,4,5,3,1
    2,5,3,1...2,5,4,1...2,5,3,4,1...2,5,4,3,1


    Starting with 2, there are 8 valid sequences

    The same type of procedure will count the others.
    Last edited by Archie Meade; March 7th 2010 at 12:01 PM.
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