Expert: Steve Holleran - 10/28/2007

Question
let P(X) = .55 and P (Y) =.35. Assume the probability that they both occur is .20.  What is the probability of either X or Y occuring?

Answer
Hi Bob,

Well the basic probability rule for this is

P(X or Y) = P(X) + P(Y) = p(X and Y), so

P(X or Y) = .55 + .35 - .20 = .90 - .20 = .70

This will be true if X and Y are independent events.  You didn't state whether they were or not, so I assumed they were.

Hope this is what you needed.
Steve