If two events, A and B, cannot happen at the same time, they are called mutually exclusive.

The probability of A or B happening is the sum of their individual probabilities.

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

**Example**

A bag contains 4 red counters, 5 blue counters and 6 yellow counters.

One counter is selected at random.

What is the probability that it is red or yellow?

**Answer**

P(red) = 4/15

P(yellow) = 6/15

P(red or yellow) = 4/15 + 6/15 = 10/15 = **2/3**