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Tree Diagrams

Combining independent and dependent events with tree diagrams, with and without replacement.

  • 9 key terms
  • All boards

Learning Objectives

  1. 1Draw and complete a tree diagram for two or more events.
  2. 2Multiply along the branches for "and", and add the outcomes for "or".
  3. 3Solve problems with independent events, and with dependent events such as picking without replacement.
  4. 4Use "at least one" as 1 minus the probability of none.

Tracking two events

When two or more things happen one after the other, a tree diagram shows every possible path, with the probability written on each branch. It stops you missing outcomes, and it gives you a method that always works: multiply the probabilities along a path to get the chance of that path, and add the paths that match what you want. Tree diagram questions are among the most common on a non-calculator paper, usually with fractions that cancel nicely.

Independent events

Two events are independent if the first does not change the probability of the second, such as two throws of a coin, or picking a counter and then putting it back.

  • Same probabilities on each branch

    The second set of branches has the same probabilities as the first.

  • The branches from one point add up to 1

    \(0.3 + 0.7 = 1\).

  • And means multiply

    The probability of rain on both days is \(0.3 \times 0.3 = 0.09\).

  • Or means add

    Add the end results that match what the question wants.

At least one

The probability that it rains on any day is 0.3. Assuming the days are independent, work out the probability that it rains on at least one of two days.

Show the solutionHide the solution
  1. 1 Find the probability of the opposite The opposite of "at least one" is "none": no rain on both days.
  2. 2 Multiply \(0.7 \times 0.7 = 0.49\).
  3. 3 Subtract from 1 \(1 - 0.49 = 0.51\).
  4. 4 Check with the paths \(0.09 + 0.21 + 0.21 = 0.51\).

Answer0.51

Dependent events

When an item is taken and not replaced, the numbers change for the second pick, so the second probabilities depend on the first.

  • Reduce the total

    After one counter is removed, there is one fewer counter in the bag.

  • Reduce the right colour

    If the first counter was red, there is one fewer red counter too.

  • Branches still add to 1

    \(\dfrac{2}{4} + \dfrac{2}{4} = 1\).

  • Take care

    The second probabilities on each branch are different, not the same as the first.

Without replacement

A bag contains 3 red counters and 2 blue counters. Two counters are taken at random without replacement. Work out the probability that they are different colours.

Show the solutionHide the solution
  1. 1 Red then blue \(\dfrac{3}{5} \times \dfrac{2}{4} = \dfrac{6}{20}\).
  2. 2 Blue then red \(\dfrac{2}{5} \times \dfrac{3}{4} = \dfrac{6}{20}\).
  3. 3 Add the two paths \(\dfrac{6}{20} + \dfrac{6}{20} = \dfrac{12}{20}\).
  4. 4 Simplify \(\dfrac{3}{5}\).

Answer\(\dfrac{3}{5}\)

Longer trees and finding a missing probability

The same rules work for three events, and for problems where you must find a branch.

  • Three events

    Multiply three branches. For three coin tosses, each path has probability \(\dfrac{1}{2} \times \dfrac{1}{2} \times \dfrac{1}{2} = \dfrac{1}{8}\).

  • Missing branch

    The probabilities leaving a point add up to 1. If one branch is 0.6, the other is 0.4.

  • Missing number of counters

    If \(P(\text{two reds}) = \dfrac{2}{15}\) with 4 red counters in a bag of \(n\), the equation \(\dfrac{4}{n} \times \dfrac{3}{n - 1} = \dfrac{2}{15}\) can be solved (Higher tier).

  • Check the end

    The outcome probabilities should add up to 1.

Test yourself

  1. 1

    What do you do along the branches of a tree diagram?

    Show answerHide answer

    Multiply the probabilities.

  2. 2

    What do you do to combine different paths that give the outcome you want?

    Show answerHide answer

    Add their probabilities.

  3. 3

    What is \(P(\text{at least one})\)?

    Show answerHide answer

    \(1 - P(\text{none})\).

  4. 4

    What changes for the second pick when there is no replacement?

    Show answerHide answer

    The total number, and the number of the colour picked first.

  5. 5

    Two coins are tossed. What is the probability of two heads?

    Show answerHide answer

    \(\dfrac{1}{2} \times \dfrac{1}{2} = \dfrac{1}{4}\).

Exam technique: tree diagrams

Write on every branch, because the working is where the marks are.

  • Label every branch

    Write the outcome and its probability on each.

  • Check the branches

    Each pair leaving a point should add to 1.

  • Show the multiplication

    Write \(\dfrac{3}{5} \times \dfrac{2}{4}\) before simplifying.

  • Use the opposite

    For "at least one", subtracting from 1 is nearly always quicker.

Summary and exam focus

  • A tree diagram shows all the outcomes, with probabilities written on the branches.
  • Multiply along a path for "and", and add paths for "or".
  • Without replacement, the numbers change on the second set of branches.
  • \(P(\text{at least one}) = 1 - P(\text{none})\).

Exam focus

A bag contains 4 red counters and 6 blue counters. Two counters are taken at random without replacement. Work out the probability that both counters are blue. (2 marks) (2 marks)

The first counter is blue with probability \(\dfrac{6}{10}\), and then there are 5 blue counters among 9 left, so \(\dfrac{6}{10} \times \dfrac{5}{9} = \dfrac{30}{90} = \dfrac{1}{3}\). Show the two fractions before multiplying.

Key terms

The words this lesson expects you to use. Each one is linked from the first place it appears above.

Tree diagram
A diagram with branches that shows the outcomes of events in order and their probabilities.
Branch
A line on a tree diagram showing one possible outcome and its probability.
Independent events
Events where one does not affect the probability of the other.
Dependent events
Events where the first changes the probability of the second.
Replacement
Putting an item back before the next pick.
Without replacement
Not putting an item back, so the next pick has fewer items.
At least one
One or more, found as 1 minus the probability of none.
Outcome probability
The probability of one complete path through the tree.
Product
The result of multiplying.

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