Edexcel IGCSE Maths topic guide

Probability

Probability work becomes reliable when every outcome and branch is labelled. Multiplication follows a path through stages; addition combines mutually exclusive paths that satisfy the event.

Foundation and Higher relevance

Foundation and Higher both cover probability scales and combined events. Higher questions make more use of conditional probability, algebraic probabilities and set notation.

Topic coverage

Probability scale
Basic probability and relative frequency
Mutually exclusive and independent events
Sample-space diagrams
Tree diagrams with and without replacement
Conditional probability
Set notation and Venn diagrams

Worked example

Question: A bag contains 3 red and 2 blue counters. Two counters are taken without replacement. Find the probability both are red.

The first red probability is 3/5.
After one red is removed, 2 red counters remain from 4 counters.
Multiply along the path: 3/5 × 2/4.

Answer: The probability is 6/20 = 3/10.

Common errors

Keeping the same denominator for a without-replacement second draw.
Adding probabilities along one path instead of multiplying.
Treating mutually exclusive events as independent events.

Revision checklist

I label every branch and make branches from one point sum to 1.
I update probabilities after an item is not replaced.
I can translate between words, tree diagrams and set notation.