GCSE Maths Revision Guide

Probability Trees

Calculate probabilities of multiple events using tree diagrams. This free GCSE Maths module combines short explanations, worked examples, flashcard-style recall, and timed practice so students can revise the topic without creating an account.

Foundation Skills

The Branches

Each set of branches from a point must add up to 1.

Example: P(Rain) = 0.3 → P(No Rain) = 0.7

Along the Branches

To find the probability of one event AND another, MULTIPLY along the branches.

Example: 0.5 × 0.5 = 0.25

Choosing Outcomes

Identify the specific end-points that satisfy the question (e.g. "at least one red").

Higher Skills

Dependent Events

The second set of branches changes based on the first outcome (e.g. picking without replacement).

At the End of Branches

To find the probability of one outcome OR another, ADD the results from the end of the trees.

Formula: P(A or B) = P(A) + P(B)

The "At Least One" Trick

P(At least one) = 1 − P(None). This is often much faster than adding many branches.