1. Not finding help here? Sign up for a free 30min 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!

Probability formula help

  1. Sep 12, 2007 #1
    1. The problem statement, all variables and given/known data

    P(A) = .7
    P(B) = .5
    P( [A U B]' ) = .1

    Find: P(B|A)

    The answer is 3/7


    2. Relevant equations

    P(B|A) = [P(B intersection A)] / [P(B)]

    3. The attempt at a solution

    I know the formula:

    P(B|A) = [P(B intersection A)] / [P(B)]

    but how do I find P(B intersection A)] ?

    any help would be greatly appreciated.

    Thanks
     
  2. jcsd
  3. Sep 12, 2007 #2
    Is there a formula for [itex]P(A \cup B)[/itex] that has [itex]P(A \cap B)[/itex] in it?
     
  4. Sep 12, 2007 #3
    Yeah,


    P(A U B) = P(A) + P(B) - P(A intersect B)
     
  5. Sep 12, 2007 #4
    That's the one. Now how can you find P(A U B) with the information you are given?
     
  6. Sep 12, 2007 #5
    i dont know. Thats what i got stuck on
     
  7. Sep 12, 2007 #6
    I need P(B intersect A)

    P(B intersect A) = P(A) + P(B) - P(B U A)


    but i dont know how to find P(B U A)
     
  8. Sep 12, 2007 #7
    If you knew P(X') could you find P(X)?
     
  9. Sep 12, 2007 #8
    Yeah,

    P(X') = 1 - P(X)
     
  10. Sep 12, 2007 #9
    And you are given that P([A U B]') = .1, so what is P([A U B])? Now what is P(A intersect B), and finally what is P(B|A)?

    edit... You wrote the formula for P(B|A) incorrect. It should be [tex]P(B|A) = \dfrac{P(A\cap B)}{P(A)}[/tex].
     
    Last edited: Sep 12, 2007
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Probability formula help
  1. Help probability (Replies: 2)

  2. Help in Probability (Replies: 3)

Loading...