Addition Rule
Probability of A or B.
Key idea
Probability of A or B.
In depth
P(A or B) = P(A) + P(B) − P(A and B). The subtraction avoids double counting outcomes in both events.
Worked example
Card is a heart or a king: 13/52 + 4/52 − 1/52 = 16/52 = 4/13.
Formula
Real-world uses
- Finding the chance a customer buys tea or coffee when some buy both
Common mistake
Forgetting to subtract the overlap.
How to learn this
- Read the key idea and say it in your own words.
- Work through the worked example, then try a similar problem.
- Use the interactive lesson’s visual and mini-quiz to check the pattern.
Frequently asked questions
What is Addition Rule?
Probability of A or B. It is a topic at the advanced level in Probability & Statistics.
What is the formula for Addition Rule?
The key formula is P(A∪B) = P(A)+P(B)−P(A∩B).
Where is Addition Rule used in real life?
Real-world examples: Finding the chance a customer buys tea or coffee when some buy both.
What should I learn before Addition Rule?
Make sure you are comfortable with Combinations, the previous topic in Probability & Statistics.
What is a common mistake with Addition Rule?
Forgetting to subtract the overlap.
How can I practise Addition Rule?
Open the interactive lesson, play with the visual, then answer the three-question mini-quiz. Getting two right marks the topic complete.
What comes after Addition Rule?
Move on to Mutually Exclusive Events next.
Related advanced topics in Probability & Statistics
- Quartiles and IQR – Quartiles split data into four parts; IQR = Q3 − Q1.
- Box Plots – Five-number summary visualised.
- Outliers – Values beyond 1.5 IQR from the quartiles.
- Variance – Average squared deviation from the mean.
- Standard Deviation – Square root of variance; typical spread.
- Sample vs Population Variance – Divide by n−1 for sample variance.