Hello othninI think a probability tree is the easiest way to understand this. See the attached diagram.

The four end-points describe, with their respective probabilities, the possible outcomes. If you want to know the probability of arriving at more than one of these, just add their probabilities together.

So, for instance, the probability of one ofaorbbut not both is 0.5 x 0.05 + 0.5 x 0.95 = 0.5; and the probability ofaorbor bothis 0.5 x 0.95 + 0.5 x 0.05 + 0.5 x 0.95 = 0.975.

Does that make it clearer?

Grandad