and in this case i give you the exact equation for relating the probability of the union of three events to the probability of a1, a2 and a3 and so works out to be.71 but then there's all the other stuff that a1 intersect a2, a1 intersect a3, a2 intersect a3 and then you have to add in the triple intersect in a1 intersect a2 intersect a3.