Probability and statistics
Lesson 7 of 14
Probability trees
Objective. Calculate probabilities for two-stage events.
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)?
- 1First red: 3/8.
- 2Second red: 2/7.
- 33/8 × 2/7 = 6/56.
Answer: 3/28
Practice
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