Question

Events A and B are disjointed.

P(A) = 1/10

P(B) =3/10
Find P(A or B).

Answer 1

If they are disjoint events, then the probability of "or" means that you add the two probabilities together. P(a or b) = Prob A + Prob B - Prob (A and B). Since the Prob (A and B) = 0 (since they are disjoint meaning that they can't happen together), the probabilities just add.