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Probability Question

  1. Aug 5, 2008 #1
    1. The problem statement, all variables and given/known data

    A box of 10 flashbublbs contains 3 defective bublbs. A random sample of 2 is selcted and tested. Let X be the radom variable associated with the number of defective bulbs in the sample.

    A) Find the probability distribution of X.

    B) Find the expected number of defective bulbs in a sample.

    I did this problem, I got 7/15 probability of there being no defective light bulbs, 1/15 of there being 2, but I got 7/30 of there being 1. The 7/30 is wrong but I don't know where I'm going wrong, please give me some help. Thanks in advance.

    2. Relevant equations




    3. The attempt at a solution

    7/10*6/9 = 7/15
    7/10*3/9 = 7/30
    3/10*7/9 = 1/15
     
  2. jcsd
  3. Aug 5, 2008 #2

    tiny-tim

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    Hi Sucks@Physics ! :smile:

    (in the last line, you wrote a 7 instead of a 2 :wink:)

    As you've probably noticed, the three probabilities should add up to 1.

    Your 7/30 is the probability of the first one being defective, and the second one good … but you also need the probability of the first one being good, and the second one defective … that doubles it! :smile:
     
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