The probability P(A+B) that at least one of the events A and B
occurs is given by:

P(A+B) < or = P(A) + P(B)

where P(A) and P(B) are the probabilities of A and B respectively. The
equality applies if events A and B are exclusive, i.e. the
occurrence of one precludes the other.

Rule 3:

The probability P(AB) of obtaining both A and B is given by:

P(AB) = P(A/B)P(B) [or P(B/A)P(A)]

where P(A/B) is the probability of A occurring, given that B has
occurred already.