What you'll learn
How to calculate and interpret simple probabilities.
Key ideas
- Probability runs from 0 (impossible) to 1 (certain).
- Theoretical probability = favourable outcomes ÷ total equally likely outcomes.
- Experimental probability (relative frequency) = number of successes ÷ number of trials.
- Complement: P(not A) = 1 − P(A).
- Expected frequency = probability × number of trials.
Reliability
- Relative frequency gets closer to the true probability as the number of trials increases.
Exam tips
- Give probabilities as fractions, decimals or percentages — not 'ratios' or words.
- Use the complement to save work when 'at least one' appears.
Common mistakes
- Writing probability as a ratio (e.g. 1:6).
- Forgetting probabilities of all outcomes sum to 1.