Probability (KS3 introduction)

P(event) = number of favourable outcomes / total outcomes. Sample space and listing.

KS3 introduces numerical probability. P(event) is between 0 and 1. The sum of all possible outcome probabilities is 1. Sample-space diagrams list every possible outcome.

Worked examples

P(rolling 4 on a fair die) = 1/6.

P(picking a red card from a standard pack) = 26/52 = 1/2.

Two coins: sample space {HH, HT, TH, TT}; P(at least one head) = 3/4.

Frequently asked questions

0 to 1 vs 0 to 100%?

Same scale. P(A) = 0.4 = 40%.

Mutually exclusive?

Cannot happen at the same time. P(A or B) = P(A) + P(B). Independent events: P(A and B) = P(A) × P(B). KS4 develops this.