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Probability of P(A or B)

  • Thread starter g.lemaitre
  • Start date
  • #1
267
1

Homework Statement



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

Homework Equations


The Attempt at a Solution



Why isn't the answer just .9, add the two probabilities together? The book says the answer is .8
 

Answers and Replies

  • #2
267
1
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
 
  • #3
6,054
390
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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