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Topic summary · Foundation · CIE Core

Probability summary

Probability measures likelihood from 0 to 1. Listing outcomes carefully and using fractions in lowest terms keeps marks safe.

Open Learning Sheet — ProbabilityDownload Probability answers PDF

Files: foundation-12 Probability handout.pdf · foundation-12 Probability handout answers.pdf · free public downloads · diagrams in the PDFs

Key learning points

  • Theoretical probability = favourable ÷ equally likely outcomes.
  • Probabilities of mutually exclusive exhaustive events sum to 1.
  • And/or language: multiply along a branch for AND; add for mutually exclusive OR.

Worked example 1 — Simple probability

Fair die. P(prime)P(\text{prime})?

Working

P=36=12P=\tfrac{3}{6}=\tfrac{1}{2}

Primes: 2, 3, 5.

Worked example 2 — Complement

If P(A)=0.35P(A)=0.35, find P(A′)P(A').

Working

P(A′)=1−0.35=0.65P(A')=1-0.35=0.65

Complementary probabilities sum to 1.

Worked example 3 — Independent events

Two fair coins. P(two heads)P(\text{two heads})?

Working

12×12=14\tfrac{1}{2}\times\tfrac{1}{2}=\tfrac{1}{4}

Multiply independent probabilities.

Scaffolded Learning Sheet and answers (PDFs)

This page is a short, searchable overview for revision and search engines. Use the Scaffolded Learning Sheet first, then the Answers PDF for a full written solution. The printable PDFs remain the main classroom resource.

Open Learning SheetAnswers PDF
WorksheetsTopic summariesTopic videos
Probability summary — worksheet & answers PDF