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MathidiaMathematics Academy
Probability and statistics
Lesson 7 of 14
Introduction

Probability trees

Objective. Calculate probabilities for two-stage events.

  1. 1. Learn
  2. 2. Worked example
  3. 3. Practice
  4. 4. Feedback
  5. 5. Continue

Learn: the key idea

Multiply along the branches for “and”, add between branches for “or”. Branches from any point must total 1.

Worked example

A bag has 3 red and 5 blue counters. Two are taken without replacement. P(both red)?

3 … 5
Illustrated card showing 3 … 5
  1. 1First red: 3/8.
  2. 2Second red: 2/7.
  3. 33/8 × 2/7 = 6/56.

Answer: 3/28

Concept mastery

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Practice

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Your progress

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