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Homework Help: Propability: Somebodys mistake !

  1. Sep 20, 2010 #1
    1. The problem statement, all variables and given/known data
    bayes theorem
    we got 5 boxes with white and black balls ,2 boxes have 2 white balls and 3 black, 2 boxes have 1 white and 4 black , 1 box has 4 whites and 1black .From a random extraction the ball turned out to be black find propability that the extracted ball was from the second box

    p(h1)=2/5 ,p(h2)=2/5, p(h3)=1/5 ,p(h1)a=3/5,P(h2)a=4/5,p(h3)a=1/5

    This is the way its written in the book and professor tought us as its written in the book

    Now I know how it goes but I think there might be a problem in the book because p(h2) is not really 2/5 but it should 1/5

    am I correct or I have misunderstood all this problem ?
  2. jcsd
  3. Sep 20, 2010 #2


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    Staff: Mentor

    At first I thought it is a typo, but you are consistent in all your threads.

  4. Sep 20, 2010 #3


    Staff: Mentor

    What do h1, h2, and h3 mean? What is a?
  5. Sep 20, 2010 #4
    my tongue language I think pronounces p instead of b .I have the book right here let me check......wait a sec the book actually writes 'b' as well .Anywayz
  6. Sep 20, 2010 #5
    I think it must 'the happening or event' I dont know I hate learning literature related to math.
  7. Sep 20, 2010 #6


    Staff: Mentor

    It's impossible to determine the probability of an event if you don't know what the event is.

    Also, what is a?
  8. Sep 20, 2010 #7
    Sorry , the way it should be done is : ph1(A)=3/5, ph2(A)=4/5 ,ph3(A)=1/5

    p(h1)=2/5 2 white balls ,ph1(A)= 3/5, 3 white balls all boxes equal 5 so...
  9. Sep 20, 2010 #8


    Staff: Mentor

    I don't understand ph1(A), ph2(A), or ph3(A) either, and you still haven't explained what A is.

    Probabilities are usually written as P(some event). A typical example is when we flip a coin. One side is called the head side, and the other is the tail side. If we disallow the possibility of the coin landing on its edge, the two probabilities are P(H) and P(T), and each probability is 1/2 for a fair coin.
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