How to use it
- Find each individual probability.
- Find the overlap.
- Subtract the overlap once.
See it in action
If P(A)=0.5, P(B)=0.4, overlap=0.2, then union=0.7.
How to verify it
The union cannot be smaller than either event alone.
Add Probabilities Without Double Counting
For events A and B, P(A or B)=P(A)+P(B)−P(A and B).