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Please help me

  1. Nov 4, 2009 #1
    i have an exam after 1 week and while i was studying i found this simple problem
    but i couldnt answer it as the author
    it is

    We have two urns, I and II. Urn I contains 2 black balls and 3 white
    balls. Urn II contains 1 black ball and 1 white ball. An urn is drawn at random
    and a ball is chosen at random from it. We can represent the sample space of this
    experiment as the paths through a tree as shown in Figure 4.1. The probabilities
    assigned to the paths are also shown.
    Let B be the event “a black ball is drawn,” and I the event “urn I is chosen.”
    Then the branch weight 2/5, which is shown on one branch in the figure, can now
    be interpreted as the conditional probability P(B|I).

    any one please tell me why it is 2/5 because i think it is 1/5
    figures mentioned arenot important in solving it

    thnx for help
  2. jcsd
  3. Nov 5, 2009 #2


    User Avatar
    Science Advisor

    Why do you think it should be 1/5 and why do you think the figures are not important?

    The "branch weight" is 2/5 precisely because urn I contains 5 balls, 2 of which are black.
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