Probability · SS 2
Addition rule (OR)
Find the chance of one event OR another.
- Mutually exclusive: P(A or B) = P(A) + P(B)
- In general: P(A or B) = P(A) + P(B) − P(A and B)
What the letters mean
- A, B
- two events
- mutually exclusive
- they cannot happen together
Worked example
A fair die is thrown. Find the probability of an even number or a number greater than 4.
Step 1: The events overlap at 6
P(even) = 36, P(> 4) = 26, P(both) = 16
Step 2: Add and take off the overlap
36 + 26 − 16 = 46 = 23
Final answer: 23
'OR' → add, but only when the events can't happen together; otherwise take off the overlap. 'AND' → multiply, but only for independent events.
See it
Play with this picture from the Probability Rules lesson.
1st ↓ 2nd →
6 of 36 outcomes → P = 636 = 16