The probability of alternative occurance states: the probability that one or the other or both of two events will occur is equal to the probability that the first will occur, plus the probability that the second will occur, minus the probability that they both occur. (The final term in this formula provides a necessary correction because we have already counted the joint occurrence twice, once in each of the other terms.)

P(A V B) = P(A) + P(B) - P(A · B)

In this case we have three events. So we do it twice.

The probability that A or B will happen is A + B - A*B or .7 +.5 - (.7*.5) or .85 now we do it again with the result and C

so .85 + .65 - (.85*.65) or0.9475