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Homework Help: Finding Conditonal Probability

  1. Oct 13, 2009 #1
    The problem statement, all variables and given/known data
    There are w white balls and b black balls in a bowl. Randomly select a ball from the bowl and then return it to the bowl along with n additional balls of the same color. Another single ball is randomly selected from the bowl(now containing w+b+n balls) and it is black. Show that the conditional probability that the first ball selected was white is w/(w+b+n)

    Relevant equations
    The conditional probability of an event A, given that an event B has occurred, is equal to:

    The attempt at a solution
    This is my last question of my assignment and I can't figure out even how to get the first step. The condition we know here is an event happened afterward, so I am even confused if I should use the equation above. I try to list the sample points which are A(1st-w, 2nd-w), B(1st-w, 2nd-b), C(1st-b, 2nd-b), D(1st-b, 2nd-w), and the possible sample points should be B or D. Then I try the conditional probability for B:

    Sample point B :

    Being the first selecting:
    P(w)=w/(w+b) P(b)=b/(w+b)

    By sample point B, it supports that the first selected ball is white, second is black, so using the equation:
    so, P(bnw)=[w/(w+b)]*[b/(w+n+b)]

    Here it already looks strange because B is just one of the sample points, but I still continue:


    I got the answer, but I have no feeling for that. It was that I was just putting something into an equation with no reason. But when I tried other ways, it even went worse. If anyone can give me some ideas just like how I should start to prove and that will be great.
  2. jcsd
  3. Oct 13, 2009 #2


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    I don't understand what the question is asking: "Show that the conditional probability that the first ball selected was white is w/(w+b+n)" This doesn't make any sense. The probability that the first ball selected is white is clearly w/(w+b) unless i am horribly, horribly mistaken. Reread the question.

    EDIT: Oh never mind, I missed the part where the second ball selected was black.

    After studying this problem further, I decided I don't know enough to help you out, sorry. Hopefully someone else can explain.
    Last edited: Oct 13, 2009
  4. Oct 13, 2009 #3
    I think it means that at first the color of the ball is unknown, and given the condition that the second selected ball is black; thus, under this condition what the probability of the the first ball selected was white is.
    That is what I got. I agree with your meaning though, but anyway that is what the question is asking, driving me cruzy.
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