Apps. and Interpretation for IBDP Maths Ebook 1 | Page 282

Your Practice Set – Applications and Interpretation for IBDP Mathematics
80
Paper 2 – Problems Involving Tree Diagrams
Example
Petr travels to school on a train. On any day, the probability that Petr will miss the train is
0.2.
If he misses the train, the probability that he will be late for school is 0.9. If he does not
miss the train, the probability that he will be late is 0.25.
Let A be the event “he misses the train” and B the event “he is late for school”.
The information above is shown on the following tree diagram.
(a)
Find
(i) P( A� B)
;
(b)
(ii) P( B ) .
Find the probability that
[4]
(i)
Petr does not miss the train and is not late for school;
(ii)
Petr does not miss the train, given that he is late for school.
The cost for each day that Petr catches the train is 5 dollars. Petr goes to school on
Monday and Friday. Let X be the total transportation cost.
[5]
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