Tree Diagrams
(KS3, Year 7)
The Lesson
A tree diagram shows all the possible outcomes of an event and their
probabilities.
A Real Example of a Tree Diagram
It is easier to understand a tree diagram with an example.
Tossing a Coin
If a coin is tossed, the coin can land on
Heads or
Tails.

Each outcome has its own branch. There is a branch for Heads and for Tails.

The probability of each outcome is written by each branch. The probability of Heads (^{1}⁄_{2}) is written by the branch. The probability of Tails (^{1}⁄_{2}) is written by the branch.
Note: The probabilities across the branches
add up to 1. They are
mutually exclusive and exhaustive events.
Another Real Example of a Tree Diagram
A tree diagram can be extended to more than one event.
The tree diagram below is for two coin tosses.
What Is Probability?
A probability is a measure of how likely (how
probable) an event is to happen.
A probability is expressed as a number between 0 (impossible) and 1 (certain).
The formula for finding a probability is shown below:
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See Also
What is probability?
What are mutually exclusive events?