Maths Literacy Matric Revision: Tree diagrams

CAPS Grade 12 Mathematical Literacy: Probability Tree Diagrams Revision Notes


Tree diagrams are a visual way to calculate the probabilities of different outcomes in a sequence of events. They are useful in decomposing complex probability problems into more manageable parts, making it easier to see the relationship between different events. Tree diagrams are particularly important for understanding conditional probabilities and for visualizing the combined probabilities of independent or dependent events.

Essential Learning Objectives:

  • Understand the structure of a tree diagram.
  • Use tree diagrams to calculate probabilities of combined events.
  • Differentiate between dependent and independent events.

1. Key Points

Structure of Tree Diagrams:

  • Branches: Represent possible outcomes at each stage.
  • Nodes: Points where branches split. Each node represents an event.
  • Paths: The routes from the start node to an end node represent the sequence of outcomes.

Calculating Probabilities:

  • Probabilities along the branches from a single node must add up to 1.
  • Multiply the probabilities along a path to find the probability of a sequence of events.
  • Add probabilities of different paths to find the combined probability of two or more events.

Types of Events:

  • Independent Events: The outcome of one event does not affect the outcome of another.
  • Use multiplication rule: ( P(A \text{ and } B) = P(A) \times P(B) ).
  • Dependent Events: The outcome of one event affects the outcome of another.
  • Use conditional probabilities: ( P(A \text{ and } B) = P(A) \times P(B|A) ).

2. Real-World Applications

Example Problem:

A school’s inter-house swimming gala is held at night 60% of the time and during the day 40% of the time. If the gala is held at night, the probability of a thunderstorm is 0.8. If held during the day, the probability of a thunderstorm is 0.4. Use a tree diagram to determine the probability of a gala being postponed due to a thunderstorm.

Step-by-Step Solution:

  1. Draw the Tree Diagram:
  2. Initial Node: Gala (Night 0.6, Day 0.4).
  3. Second Nodes:

    • Night splits to Thunderstorm (0.8) and No Thunderstorm (0.2).
    • Day splits to Thunderstorm (0.4) and No Thunderstorm (0.6).
  4. Calculate Path Probabilities:

  5. Night & Thunderstorm: (0.6 \times 0.8 = 0.48).
  6. Night & No Thunderstorm: (0.6 \times 0.2 = 0.12).
  7. Day & Thunderstorm: (0.4 \times 0.4 = 0.16).
  8. Day & No Thunderstorm: (0.4 \times 0.6 = 0.24).

  9. Total Probability of Postponement:

  10. Add probabilities of Night & Thunderstorm and Day & Thunderstorm:
    (0.48 + 0.16 = 0.64).

Answer: The probability of the gala being postponed is 0.64 (64%)【4:1†source】.

3. Common Misconceptions and Errors

  • Forgetting to Normalize Branch Probabilities: Ensure the probabilities from any node add up to 1.
  • Confusing Independent and Dependent Events: Remember to check whether events influence each other.
  • Incorrect Multiplication Along Paths: Always multiply the probabilities along the branches for the sequence of events.

4. Practice and Review

Practice Questions:

  1. A bag contains 5 black caps, 2 white caps, and 4 red caps. What is the probability of selecting two caps in sequence, one white and one red?
  2. In a game, if Player A and Player B each have a 0.5 probability of winning their match, what is the probability that both win their matches?


  1. Using ( P(White \text{ and } Red) = \frac{2}{11} \times \frac{4}{10} = 0.0727 ) (7.27%).
  2. Both win: ( P(A \text{ and } B) = 0.5 \times 0.5 = 0.25 ) (25%)【4:3†source】【4:2†source】.

5. Connections and Extensions

  • Tree diagrams help in understanding joint probabilities and conditional probabilities in statistics and various real-life contexts like genetics, business decision-making, and risk management.
  • Extend knowledge by exploring Venn diagrams and two-way tables, useful for visualizing events and their probabilities.

6. Summary and Quick Review

  • Tree diagrams visually break down complex probability scenarios.
  • Multiply along paths and add different paths’ probabilities for the final answer.
  • Confirm understanding of independent versus dependent events.

7. Additional Resources:

These resources offer video tutorials and other interactive content to reinforce learning.


Tree diagrams are an invaluable tool for mastering probability problems. Mastery of creating and interpreting them can significantly improve your problem-solving skills in both academics and real-life applications.

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