Probability Tree Diagrams (Cambridge (CIE) IGCSE International Maths)

Revision Note

Tree Diagrams

How do I draw a tree diagram?

  • Tree diagrams can be used for repeated experiments with two outcomes

    • The 1st experiment has outcome A or not A

    • The 2nd experiment has outcome B or not B

  • Read the tree diagram from left to right along its branches 

    • For example, the top branches give A followed by B

      • This is called A and B

How to set up a tree diagram for two experiments each with two possible outcomes

How do I find probabilities from tree diagrams?

  • Write the probabilities on each branch

    • Remember that P(not A) = 1 - P(A)

      • Probabilities on each pair of branches add to 1

  • Multiply along the branches from left to right

    • This gives P(1st outcome and 2nd outcome)

  • Add between the separate cases

    • For example

      • P(AA or BB) = P(AA) + P(BB)

  • The probabilities of all possible cases add to 1

  • If asked to find the probability of at least one outcome, it is quicker to do 1 - P(none)

Examiner Tips and Tricks

  • When multiplying along branches with fractions, don't cancel fractions in your working - having the same denominator makes them easier to add together!

Worked Example

A worker drives through two sets of traffic lights on their way to work.
Each set of traffic lights has only two options: green or red.
The probability of the first set of traffic lights being on green is 5 over 7.
The probability of the second set of traffic lights being on green is 8 over 9.

(a) Draw and label a tree diagram. Show the probabilities of every possible outcome.

Work out the probabilities of each set of traffic lights being on red, R
Use P(red) = 1 - P(green)

straight P open parentheses 1 to the power of st space straight R close parentheses equals 1 minus straight P open parentheses 1 to the power of st space straight G close parentheses equals 1 minus 5 over 7 equals 2 over 7
straight P open parentheses 2 to the power of nd space straight R close parentheses equals 1 minus straight P open parentheses 2 to the power of nd space straight G close parentheses equals 1 minus 8 over 9 equals 1 over 9

Draw the branches (with a label of G or R on the ends)
Write the probabilities above each branch
Calculate probabilities of each outcome by multiplying along the branches from left to right

A tree diagram showing different outcomes at traffic lights

 (b) Find the probability that both sets of traffic lights are on red.

This is the answer for P(R, R) from the tree diagram

 bold 2 over bold 63

(c) Find the probability that at least one set of traffic lights are on red.

This means the 1st is green and the 2nd is red
Or the 1st is red and the 2nd is green
Or the 1st is red and the 2nd is red ('at least one' could mean both)
Add between the separate cases

straight P open parentheses at space least space one space straight R close parentheses space equals space straight P open parentheses straight G comma space straight R close parentheses space plus space straight P open parentheses straight R comma space straight G close parentheses space plus space straight P open parentheses straight R comma space straight R close parentheses space equals space 5 over 63 space plus space 16 over 63 space plus space 2 over 63 space equals space 23 over 63

bold 23 over bold 63

Alternative Method
At least one red means all the possible cases shown except two greens
So P(at least 1 red) = 1 - P(two greens)

space 1 space minus space straight P open parentheses straight G comma space straight G close parentheses space equals space 1 space minus space 40 over 63 space equals space 23 over 63 

bold 23 over bold 63

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Mark Curtis

Author: Mark Curtis

Expertise: Maths

Mark graduated twice from the University of Oxford: once in 2009 with a First in Mathematics, then again in 2013 with a PhD (DPhil) in Mathematics. He has had nine successful years as a secondary school teacher, specialising in A-Level Further Maths and running extension classes for Oxbridge Maths applicants. Alongside his teaching, he has written five internal textbooks, introduced new spiralling school curriculums and trained other Maths teachers through outreach programmes.

Dan Finlay

Author: Dan Finlay

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Dan graduated from the University of Oxford with a First class degree in mathematics. As well as teaching maths for over 8 years, Dan has marked a range of exams for Edexcel, tutored students and taught A Level Accounting. Dan has a keen interest in statistics and probability and their real-life applications.