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Math Help - Easy Probability Question

  1. #1
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    Easy Probability Question

    If P(A) = 0.2, P(A|B) = 0.3, P(A u B) = 0.4. Find P(B).

    Reminding you that:
    P(A n B) = P(A) + P(B) - P(A u B)
    P(A u B) = P(A) + P(B) - P(A n B)

    & that P(A|B) = P(A n B) / P(B)


    I think I've got to use simultaneous equations... I just can't figure out where.
    Any help? Please show working.

    Thanks
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  2. #2
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    Edit:
    Use this fact: P(B\cap A)=0.3P(B).
    & 0.4=0.2+P(B)-P(B\cap A)
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  3. #3
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    I've got it. Thanks.

    Bit slow today. Cheers.
    Last edited by Heathcliff; November 22nd 2010 at 03:45 PM.
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