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
g.lemaitre
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
voko
6,054
391
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.