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Homework Help: Probability of P(A or B)

  1. Sep 13, 2012 #1
    1. The problem statement, all variables and given/known data

    If P(A) = .4, P(B) = .5 and P(A and B) = .1, find P(A or B)

    2. Relevant equations
    3. The attempt at a solution

    Why isn't the answer just .9, add the two probabilities together? The book says the answer is .8
  2. jcsd
  3. Sep 13, 2012 #2
    I got it now. I didn't see this equation

    P(A or B) = P(A) + P(B) - P(A and B)

    .4 + .5 - .1 = .8
  4. Sep 13, 2012 #3
    That would work, if A and B were disjoint. But you have P(A and B) > 0, so there is some intersection. If you just take P(A or B) = P(A) + P(B), you count that intersection TWICE.
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