You will be asked to calculate probabilities involving conditional probability. In this next tutorial you are shown how to calculate P(A|B)

Calculating Conditional Probability in tree diagrams : ExamSolutions Maths Revision - youtube Video


Here is a typical question you should try on conditional probability. Try it before looking at the worked solution.

Susan goes to work by one of two routes A or B. The probability of going by route A is 30%.
If she goes by route A the probability of being late for school is 5% and if she goes by route B, the probability of being late is 10%.

Draw a probability tree diagram, and then

(i) Find the probability that Susan is late for school
(ii) Given that Susan is late for school, find the probability that she went via route A.

Conditional Probability Example : ExamSolutions - youtube Video