When do we use the addition rule?

If we want to calculate the probability of event A occurring or event B occurring, taking into account that they are mutually exclusive, we use the addition rule.

Formula:

P(A or B) = P(A) + P(B), where

• P(A or B) - the probability of event A occurring or event B occurring,
• P(A) - the probability of event A occurring
• P(B) - the probability of event B occurring

Task example:

The basket has 12 green balls, 30 red balls, and 18 yellow balls. What is the probability of taking a green or red ball?

1. P(green) = 12/60 = 0.2 = 20%
2. P(red) = 30/60 = 0.5 = 50%
3. P(green or red) = P(green) + P(red) = 20% + 50% = 70%.