Tree DiagramsCalculate the probability of independent and dependent combined events using tree diagrams. 
1. In a box there are five red balls and two yellow balls. On Monday Isabel picked a ball at random from the box, played with it then put it back. On Tuesday she picked a ball at random from the box.
a) Complete the tree diagram.
b) What is the probability that on both days she picked a red ball? ^{}/_{}
c) What is the probability that on both days she picked a yellow ball? ^{}/_{}
d) Calculate the probability that on Monday she picked a red ball and on Tuesday she picked a yellow ball.^{}/_{}
Red
Yellow
Red
Yellow
Red
Yellow
2. In a round tin there are six cream cakes and three plain cakes. In a square based tin there are seven cream cakes and four plain cakes. The tree diagram shows the probabilities of picking one cake from each tin.
a) Complete the tree diagram.
b) What is the probability of picking two cream cakes? ^{}/_{}
c) What is the probability of picking two plain cakes? ^{}/_{}
d) Calculate the probability of picking two different types of cake.^{}/_{}
Cream
Plain
Cream
Plain
Cream
Plain
3. In a packet there are seven lemon sweets and eight lime sweets. This morning Isabel ate one sweet from the packet before breakfast and she ate another after breakfast. Both sweets were picked randomly from what was in the packet at the time.
a) Complete the tree diagram.
b) What is the probability of picking two lemon sweets? ^{}/_{}
c) What is the probability of picking a lemon sweet followed by a lime sweet? ^{}/_{}
d) Calculate the probability of picking a lime sweet followed by a lemon sweet?.^{}/_{}
Lemon
Lime
Lemon
Lime
Lemon
Lime
4. The probability that the first bus is late is \( \frac{1}{11}\). If the first bus is late, the probability of the second bus being late is \( \frac{1}{10}\) otherwise it is \( \frac{1}{9}\) (buses are either on time or late, they are never early!).
a) Complete the tree diagram.
b) What is the probability of both buses being late? ^{}/_{}
c) What is the probability of both buses being on time? ^{}/_{}
d) What is the probability of at least one bus being on time? ^{}/_{}
Late
On Time
Late
On Time
Late
On Time
5. In a bag there are three red, four blue and five green counters. Two counters are taken out and not replaced.
a) Complete the tree diagram.
b) What is the probability of both counters being red? ^{}/_{}
c) What is the probability of both counters not being red? ^{}/_{}
d) What is the probability of at least one counter being red? ^{}/_{}
Red
Not red
Red
Not red
Red
Not red
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Level 1  Basic Tree Diagram questions
Level 2  A challenging puzzle that can be solved with the aid of a tree diagram
Level 3  GCSE and IB examstyle questions involving tree diagrams
This video explains how to use tree diagrams to solve probability questions and is from Corbettmaths.
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