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Thread: Probabilty help

  1. #1
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    Probabilty help

    I am unsure about the following problems, I put what I think the answer is, would appreciate any help. Thanks!


    1. For two events A and B, suppose P(A)= .45, P(B)=.75 and P(BlA)= .45
    Are A and B independent?

    I said no. For them to be independent, P(BlA) would have to equal .75


    2. The events A and B are mutually exclusive. If P(A)= .4 and P(B)=.5 what is P(A and B)?

    I feel like this might be a trick question? I thought mutually exclusive refered to P(A or B). If it means the same as independent then I answered
    P(A and B)=P(A)*P(B)


    3. Given P(A and B)= .25, P(A)= .20 and P(A and B)= .30 find P(B)

    Just rearranging I got:
    P(A or B)= P(A) + P(B) - P(A and B)

    P(B) = P(A or B) - P(A) + P(A and B)
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  2. #2
    MHF Contributor matheagle's Avatar
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    Quote Originally Posted by kittysnyde View Post
    I am unsure about the following problems, I put what I think the answer is, would appreciate any help. Thanks!


    1. For two events A and B, suppose P(A)= .45, P(B)=.75 and P(BlA)= .45
    Are A and B independent?

    I said no. For them to be independent, P(BlA) would have to equal .75


    2. The events A and B are mutually exclusive. If P(A)= .4 and P(B)=.5 what is P(A and B)?

    I feel like this might be a trick question? I thought mutually exclusive refered to P(A or B). If it means the same as independent then I answered
    P(A and B)=P(A)*P(B)


    3. Given P(A and B)= .25, P(A)= .20 and P(A and B)= .30 find P(B)

    Just rearranging I got:
    P(A or B)= P(A) + P(B) - P(A and B)

    P(B) = P(A or B) - P(A) + P(A and B)
    well you have three totaly different answers coming...

    1. is correct ...
    if you have indep then P(B|A)=P(B)

    2. You're wrong here...
    Indep and ME are almost opposites
    Instead use $\displaystyle P(A\cup B)=P(A)+P(B)-P(AB)=P(A)+P(B)$
    HOWEVER, you asked for P(A and B)=P(AB)=0
    ME means that the intersection is empty, hence the probability is zero.

    3. Doesn't make sense
    $\displaystyle .25\ne.3$

    YOU seem to be mixing up and's (which are intersections) with or's (unions).
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