1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Homework Helper

    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


    User Avatar
    Homework Helper

    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" ?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook