1. Sep 13, 2012 #1

   g.lemaitre

   

   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

    

   g.lemaitre, Sep 13, 2012
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3. Sep 13, 2012 #2

   g.lemaitre

   

   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

    

   g.lemaitre, Sep 13, 2012
4. Sep 13, 2012 #3

   voko

   

   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.

    

   voko, Sep 13, 2012

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