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June 2016 p62 q1
2611
Ayman’s breakfast drink is tea, coffee or hot chocolate with probabilities 0.65, 0.28, 0.07 respectively. When he drinks tea, the probability that he has milk in it is 0.8. When he drinks coffee, the probability that he has milk in it is 0.5. When he drinks hot chocolate he always has milk in it.
(i) Draw a fully labelled tree diagram to represent this information.
(ii) Find the probability that Ayman’s breakfast drink is coffee, given that his drink has milk in it.
Solution
(i) The tree diagram is constructed as follows:
First branch: Tea (T) with probability 0.65, Coffee (C) with probability 0.28, Hot Chocolate (HC) with probability 0.07.
Second branch for Tea: Milk (M) with probability 0.8, No Milk (NM) with probability 0.2.
Second branch for Coffee: Milk (M) with probability 0.5, No Milk (NM) with probability 0.5.
Second branch for Hot Chocolate: Milk (M) with probability 1, No Milk (NM) with probability 0.
(ii) To find the probability that Ayman’s breakfast drink is coffee, given that his drink has milk in it, we use Bayes' theorem: