1. Let A and B be mutually exclusive events. Then the probability of their union is the sum of their individual probabilities;
More generally, if are mutually exclusive events, then
2. If are collectively exhaustive events, then the probability of their union is .
Since the events are collectively exhaustive, their union is the whole sample space S and .
A drawer contains three pairs of red socks, two pairs of black socks and four pairs of brown socks. If a person in a dark room selects a pair of socks, find probability that the pair will be either black or brown. (Note: The socks are folded together in matching pairs).
Let us define the following events
A= the selected socks are black
B= the selected socks are brown.
Since there are nine pairs of socks,
P (black) =P (A) = ; P (brown) =P (B) =
P(black or brown)= .
A day of the week is selected at random. Find the probability that it is a weekend day.
A= the selected day is Saturday
B= the selected day is Sunday
P(A)= ; P(B)= and