# Probability Tree Diagram

Here we will learn about probability tree diagrams, including what they are and how to complete them. We will also look at calculating probabilities using probability tree diagrams.

There are also probability tree diagram worksheets based on Edexcel, AQA and OCR exam questions, along with further guidance on where to go next if you’re still stuck.

## What are probability tree diagrams?

Probability tree diagrams are a way of organising the information of two or more probability events. Probability tree diagrams show all the possible outcomes of the events and can be used to solve probability questions.

To use tree diagrams, we need to know the probability of individual events occurring and use the fact that probabilities on each set of branches add up to \bf{1}.

Probability tree diagrams start by showing the possible outcomes for the first event, with the outcomes at the ends of the branches and the probabilities written along the branches.

The probabilities of the events can be written as fractions or decimals

For example,

A coin is flipped and a dice is rolled.

What is the probability of getting a ‘tail’ and a ‘6’?

The first event is flipping the coin. The two possible outcomes are ‘heads’ and ‘tails’. These are mutually exclusive events. They cannot happen at the same time.

The second event is rolling the dice. The possible outcomes are ‘1’, ‘2’, ‘3’, ‘4’, ‘5’ and ‘6’. However, the question is only interested in ‘6’, so we can have a ‘6’ branch and a ‘not a 6 ’ branch.

These outcomes can occur whether the coin landed on heads or tails so we add these outcomes to the end of both branches in order to show all possible combinations of outcomes.

The probability of getting a ‘6’ is \frac{1}{6}.

The probability of getting ‘not a 6 ’ will be 1-\frac{1}{6} = \frac{5}{6} .

Remember that the probabilities on each set of branches add up to \bf{1} .

We want the probability of getting a tail and a 6 so we follow the path that shows tail and 6.

The AND rule for probability states that \text{P (A and B) }=\text{P(A)} \times \text{P(B)} .

Taking the probabilities from the corresponding branches of the tree diagram, we get

Probability of getting a ‘tail’ and a ‘6’ is

\frac{1}{2}\times \frac{1}{6}=\frac{1}{12} .

Tree diagrams can be used for both independent and dependent events.

The events ‘flipping a coin’ and ‘rolling a dice’ are independent events – where the outcome of one event does not affect the outcome of the other event.

Events can also be dependent events – where the outcome of one event depends upon what has happened before.

## How to use a tree diagram to find probability

In order to use a tree diagram to find probability:

1. Fill in the probabilities on the branches.
2. Consider which outcomes are required to answer the question.
3. Find the probability of those outcomes by multiplying along the branches.
4. Use the probability/probabilities you have calculated to answer the question.

## Probability tree diagram examples

### Example 1: two independent events

A spinner is spun twice. It can land on red or it can land on blue.

Complete the tree diagram.

Work out the probability that both spins will land on blue.

1. Fill in the probabilities on the branches.

We have been given the probability of the spinner landing on red. We can work out the probability of the spinner landing on blue by using the fact that the probabilities on each set of branches add up to 1.

P(\text{blue})=1-P(\text{red})=1-\frac{1}{5}=\frac{4}{5}

Since the two spins are independent we can use the same probabilities for the second set of branches.

2Consider which outcomes are required to answer the question.

We can write the different outcomes at the ends of the branches. The question asks about two blues so we will need to look for the path which shows two blues.

3Find the probability of those outcomes by multiplying along the branches.

We need to use the probabilities on the branches and multiply them together to find the required probability.

The probability of the spinner landing on blue twice is

\begin{aligned} P(\text{blue and blue})&=P(\text{blue})\times P(\text{blue})\\\\ &=\frac{4}{5}\times \frac{4}{5}\\\\ &=\frac{16}{25} \end{aligned} .

4Use the probability/probabilities you have calculated to answer the question.

The probability that both spins will land on blue is \frac{16}{25}.

### Example 2: two independent events

In a bag there are 10 balls. There are 4 red balls and the remaining balls are green.

A ball is removed at random and the colour noted.

The ball is replaced.

A second ball is removed at random and the colour is noted.

Complete the tree diagram.

Work out the probability that there will be one ball of each colour.

Fill in the probabilities on the branches.

Consider which outcomes are required to answer the question.

Find the probability of those outcomes by multiplying along the branches.

Use the probability/probabilities you have calculated to answer the question.

### Example 3: two independent events

Mary has to catch 2 buses to work. The probability the first bus will be late is 0.1 and the probability the second bus will be late is 0.3.

Complete the tree diagram.

Work out the probability that at least one bus will be late.

Fill in the probabilities on the branches.

Consider which outcomes are required to answer the question.

Find the probability of those outcomes by multiplying along the branches.

Use the probability/probabilities you have calculated to answer the question.

### Example 4: dependent events

There are 9 sweets in a bag. 3 of the sweets are mints and the remaining sweets are chews. A sweet is taken out at random and is eaten.

A second sweet is taken at random and is also eaten.

Complete the tree diagram.

Work out the probability that two mints are eaten.

Fill in the probabilities on the branches.

Consider which outcomes are required to answer the question.

Find the probability of those outcomes by multiplying along the branches.

Use the probability/probabilities you have calculated to answer the question.

### Example 5: dependent events

In a bag there are 7 counters. There are 2 black counters and the remaining counters are white.

A counter is removed and the colour noted.

The counter is NOT replaced.

A second counter is removed and the colour is noted.

Complete the tree diagram.

Work out the probability that there will be one counter of each colour picked.

Fill in the probabilities on the branches.

Consider which outcomes are required to answer the question.

Find the probability of those outcomes by multiplying along the branches.

Use the probability/probabilities you have calculated to answer the question.

### Example 6: three independent events

Three coins are flipped.

Complete the tree diagram.

Work out the probability of getting 3 heads.

Fill in the probabilities on the branches.

Consider which outcomes are required to answer the question.

Find the probability of those outcomes by multiplying along the branches.

Use the probability/probabilities you have calculated to answer the question.

### Common misconceptions

• Cancelling fractions

It is not worth cancelling fractions when working within probability questions. This is because the numerator and denominator give information about the event, for example the number of balls in a bag. We often need to add fractions and they need a common denominator. Only cancel right at the end of a question.

• Multiplying decimals

Take care when multiplying decimals. It is easy to make mistakes. For example 0.3 \times 0.4=0.12, but some people might think the answer is 1.2, which would be incorrect.

• Dependent events

Remember that for dependent events, the probability of the second event changes depending on the outcome of the first event.

### Practice probability tree diagram questions

1. A spinner has green sections and blue sections.

The probability of the spinner landing on green is \frac{2}{7}.

The spinner is spun twice.

Using the tree diagram, work out the probability of the spinner landing on green twice.

\frac{2}{7}

\frac{4}{49}

\frac{5}{7}

\frac{4}{14}

\begin{aligned} P(\text{2 Greens})&=P(\text{Green and green})\\\\ &=P(\text{Green})\times P(\text{Green})\\\\ &=\frac{2}{7}\times \frac{2}{7}\\\\ &=\frac{4}{49} \end{aligned}

2. In a bag there are 10 balls. There are 2 red balls and the remaining balls are yellow.

A ball is removed at random and the colour noted.

The ball is replaced.

A second ball is removed at random and the colour is noted

Using the tree diagram, work out the probability that there will be a ball of each colour chosen.

0.64

0.2

0.16

0.32

\begin{aligned} P(\text{One of each colour})&=P(\text{R,Y}) \ or \ P(\text{Y,R})\\\\ &=P(\text{R,Y}) + P(\text{Y,R})\\\\ &=(0.2\times 0.8)+(0.8\times 0.2)\\\\ &=0.16+0.16\\\\ &=0.32 \end{aligned}

3. A football team wins its matches with a probability of 0.7.

Using a tree diagram, find the probability that they win at least 1 of their next two matches.

0.49

0.3

0.42

0.91
\begin{aligned} P(\text{At least one win})&=P(\text{Win, Win}) \ or \ P(\text{Win, Not win}) \ or \ P(\text{Not win, Win})\\\\ &=P(\text{Win, Win}) + P(\text{Win, Not win})+P(\text{Not win, Win})\\\\ &=(0.7\times 0.7)+(0.7\times 0.3)+P(0.3\times 0.7)\\\\ &=0.49+0.21+0.21\\\\ &=0.91 \end{aligned}

4. There are 6 chocolates in a box. 4 of the chocolates are milk chocolates and the remaining chocolates are plain chocolates. A chocolate is taken out at random and is eaten.

A second chocolate is taken at random and is also eaten.

Work out the probability that two milk chocolates are eaten.

\frac{2}{6}

\frac{3}{5}

\frac{2}{5}

\frac{13}{36}

As the first chocolate is eaten, the probabilities on the second set of branches are different.

\begin{aligned} P(\text{Two Milks})&=P(\text{Milk and Milk})\\\\ &=P(\text{Milk}) \times P(\text{Milk})\\\\ &=\frac{4}{6}\times \frac{3}{5}\\\\ &=\frac{12}{30}\\\\ &=\frac{2}{5} \end{aligned}

5. In a bag there are 8 counters. There are 3 blue counters and the remaining counters are yellow.

A counter is removed at random and the colour noted.

The counter is NOT replaced.

A second counter is removed at random and the colour is noted.

Using the tree diagram, work out the probability that there will be at least one blue counter picked.

\frac{9}{14}

\frac{15}{28}

\frac{5}{14}

\frac{3}{28}

\begin{aligned} P(\text{At least one blue})&=P(\text{B,B}) \ or \ P(\text{B,Y}) \ or \ P(\text{Y,B})\\\\ &=P(\text{B,B}) + P(\text{B,Y})+P(\text{Y,B})\\\\ &=(\frac{3}{8}\times\frac{2}{7})+(\frac{3}{8}\times \frac{5}{7})+P(\frac{5}{8}\times \frac{3}{7})\\\\ &=\frac{6}{56}+\frac{15}{56}+\frac{15}{56}\\\\ &=\frac{36}{56}\\\\ &=\frac{9}{14} \end{aligned}

6. Three dice are rolled.

Using the tree diagram, work out the probability of getting 3 even numbers.

0.125

0.5

0.25

0.333

The probability of rolling an odd number is 0.5. The probability of rolling an even number is also 0.5.

\begin{aligned} P(\text{3 Even numbers}) &= P(\text{Even and Even and Even}) \\\\ &= P(\text{Even}) \times P(\text{Even})\times P(\text{Even}) \\\\ &=0.5\times 0.5\times 0.5\\\\ &=0.125 \end{aligned}

### Probability tree diagram GCSE questions

1. A bag contains only black and white counters.

Grace picks a counter at random and then replaces it.

Grace then picks a second counter.

(a) Complete the tree diagram.

(b) Work out the probability that Grace picks 2 black counters.

(4 marks)

(a)

For \frac{2}{7} on the first branch.

(1)

For \frac{2}{7}, \frac{5}{7}, \frac{2}{7} on the second branches.

(1)

(b)

\frac{5}{7}\times \frac{5}{7}

(1)

\frac{25}{49}

(1)

2. Amir has two bags.

In the first bag there are 3 red counters and 2 blue counters.

In the second bag there are 4 red counters and 5 blue counters.

Amir takes at random a counter from the first bag.

He then takes at random a counter from the second bag.

(a) Complete the tree diagram.

(b) Work out the probability that Amir takes two blue counters.

(4 marks)

(a)

For \frac{2}{5} on the first branch.

(1)

For \frac{5}{9}, \frac{4}{9}, \frac{5}{9} on the second branches.

(1)

(b)

\frac{2}{5}\times \frac{5}{9}

(1)

\frac{10}{45}

(1)

3. Suzy eithers travels by bus or walks to the shops.

The probability that she catches a bus to the shops is 0.2.

The probability that she catches a bus from the shops is 0.9.

(a) Complete the tree diagram.

(b) Work out the probability that Suzy walks at least one way.

(5 marks)

(a)

For 0.8 on the first branch.

(1)

For 0.9 and 0.1 on the second branches.

(1)

(b)

\begin{aligned} &0.2\times 0.1=0.02 \\ &0.8\times 0.9=0.72 \\ &0.8\times 0.1=0.08 \end{aligned}

(1)

0.02+0.72+0.08

(1)

0.82

(1)

## Learning checklist

You have now learned how to:

• Complete probability trees
• Use probability trees to calculate probability of 2 or more events happening
• Use probability trees to solve probability problems

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