What is the addition rule for mutually exclusive events?

If events A and B are mutually exclusive of each other, then:
${\displaystyle P\left(A\text{or}B\right)=P\left(A\right)+P\left(B\right)}$
Mutually exclusive means that A and B cannot occur at the same time, which means P(A and B) = 0.
For example, with a single six-sided die, the probability that you roll a "4" in a single roll is mutually exclusive of rolling a "6" on that same roll because a single die can only show 1 number at a time.
In this example, if event A is rolling a "6" and event B is rolling a "4", then P(A) = 1/6 and P(B) = 1/6, and the addition rule for these two mutually exclusive events is:
${\displaystyle \frac{1}{6}+\frac{1}{6}=\frac{1}{3}}$
Thus, the addition of these two events equals ${\displaystyle \frac{1}{3}}$