# What is the key word that indictes that the Addition Rule for Probabilities will be used?

If two events, $A$ and $B$, are mutually exclusive, the probability of either one or another to happen equal to a sum their probabilities (notice different symbols used to describe logical condition $\mathmr{and}$:
$P \left(A \mathmr{and} B\right) = P \left(A + B\right) = P \left(A \cup B\right) = P \left(A\right) + P \left(B\right)$