Probability (GCSE)

P(event) = favourable / total. AND for independent: multiply. OR for mutually exclusive: add.

GCSE probability uses fractions, decimals or percentages. Sum of probabilities of all outcomes = 1. Mutually exclusive (cannot both happen) = use OR (add). Independent (one doesn't affect the other) = use AND (multiply).

Worked examples

P(red on a fair die roll) where faces are red, blue, green: depends on colour mapping.

P(red AND red, two draws with replacement, P(red)=½) = ¼.

P(rain OR snow) where mutually exclusive, 30% and 10%: 40%.

Frequently asked questions

Without replacement?

Probabilities change between draws. Tree diagram method handles this.

Sample space diagram?

Grid showing every combination of two events (e.g. two-die outcomes). Useful for AND probabilities visually.