Answer
The addition rule for probability is only true for mutually exclusive events.
Two events are disjoint/mutually exclusive if they cannot occur at the same time.
For example, when rolling two dice the events "sum is even" and "the sum is less than 5" are not disjoint (because of the sum of 4 falls in both events) and thus the addition rule cannot be used for these events.
Work Step by Step
The addition rule for probability is only true for mutually exclusive events.
Two events are disjoint/mutually exclusive if they cannot occur at the same time.
For example, when rolling two dice the events "sum is even" and "the sum is less than 5" are not disjoint (because of the sum of 4 falls in both events) and thus the addition rule cannot be used for these events.
