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Homework Help: Probabality question involving conditional probabality

  1. Sep 18, 2010 #1
    The probability of surviving a certain operation is .55. If a patient survives the operations, the propability that his or her body will reject the transplant within a month is .20. What is the probability of surviving both these critical stages.

    I am going to say P(A) = .55 and P(B | A ) = .20

    They want to know what is the probability that P(A and B) right?

    So, P(B | A) = P(A and B) / P(A)
    so P(A) * P(A|B) = P( A and B) .
    so P(A and B) = .55(.20) = .11. But the answer in the back of my book is saying .44

    Can someone explain why I am not getting the right answer?
  2. jcsd
  3. Sep 18, 2010 #2


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    The key phrase is " surviving both of these critical steps".

    What, exactly, does [itex] P(B | A) = 0.20 [/itex] the conditional probability of? (Say it in words and see if it is the value you want to use.)
  4. Sep 18, 2010 #3
    First of all, the part in bold is incorrect.

    You put P(A|B) = P(B|A)
  5. Sep 18, 2010 #4


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    It would seem to be a typo by the OP, as the original numbers created were used with the original meaning.

    The question is still: is it [itex] 0.20 [/itex] that should be used?
  6. Sep 18, 2010 #5


    If the persons body rejects the organ does he/she "survive" ?
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