## Precalculus (6th Edition) Blitzer

For two events that are not mutually exclusive, it is better to know the $and$ probability before calculating the $or$ probability. For example, rolling an even number (2,4,6) from a die has a probability of $\frac{1}{2}$; rolling a number less than 3 (1,2) has a probability of $\frac{1}{3}$; and rolling an even number less than 3 (2) has a probability of $\frac{1}{6}$. Thus, rolling an even number or less than 3 has a probability of $\frac{1}{2}+\frac{1}{3}-\frac{1}{6}=\frac{2}{3}$