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Probability problem

  1. Feb 29, 2004 #1
    Here's the question:
    A box contains 6 good and 8 defective light bulbs. The bulbs are drawn out one at a time, without replacement, and tested. What is the probability that the fifth good item is found on the ninth test?

    Could someone explain how I would go about solving this problem? Thanks!!
  2. jcsd
  3. Feb 29, 2004 #2


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    If I told you the probability that exactly 4 good items have been found within 8 tests was 0.68, could you solve the problem?

    (p.s. 0.68 is probably wrong)
  4. Mar 6, 2004 #3
    Consider the first 9 balls, this can be done 14C9 ( from 14 choose 9 its on your calculator). If the 9th ball is the 5th good then the first 9 balls must consist of 5 good and 4 bad balls.
    The probability of this happening is 6C5*8C4/14C9. If this is true you need the 9th ball to be good. This has probably 5/9.
    So the probability is 6C5*8C4/14C9 * 5/9.
  5. Mar 6, 2004 #4

    matt grime

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    Not sure about that answer, Damned.

    You want 4 good and 4 bad on the first 8, then to draw a bad on the 9th, which is to draw on of the 4 remaining bad ones from the 6 that are left.


    but they might well be the same after simplifying
    Last edited: Mar 6, 2004
  6. Mar 6, 2004 #5
    The fifth good item has to found on the 9th test. So you should replace the 4/6 with a 2/6 and this can be rearranged to give my answer. You solution is slighty better and more consistent with student examples of negative binomial etc.
    Last edited: Mar 6, 2004
  7. Mar 6, 2004 #6

    matt grime

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    Sorry for switching things over, and yes I agree with your answer entirely now I've thought about it for a second. I also agree that such conditional probabilities would be beyond the scope of the course I imagine the OP is doing.
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