## Precalculus (6th Edition) Blitzer

If it is possible for events $A\ \text{ and }\ B$ to occur simultaneously, then $P\left( A\,\text{or }B \right)$ is $P\left( A \right)+P\left( B \right)-P\left( A\text{ and }B \right)$.
We know that if $A\ \text{ and }\ B$ are not mutually executive events, the probability that either $A\ \text{or}\ B$ will occur is determined by adding their individual probabilities and also subtracting the probability that $A\ \text{ and }\ B$ occur simultaneously. Thus, the probability is: $P\left( A\,\text{or }B \right)=P\left( A \right)+P\left( B \right)-P\left( A\text{ and }B \right)$