Exam-Style Problems

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Nov 2022 p52 q1
2637

On any day, Kino travels to school by bus, by car or on foot with probabilities 0.2, 0.1 and 0.7 respectively. The probability that he is late when he travels by bus is \(x\). The probability that he is late when he travels by car is \(2x\) and the probability that he is late when he travels on foot is 0.25.

The probability that, on a randomly chosen day, Kino is late is 0.235.

(a) Find the value of \(x\).

(b) Find the probability that, on a randomly chosen day, Kino travels to school by car given that he is not late.

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Nov 2009 p62 q3
2638

Maria chooses toast for her breakfast with probability 0.85. If she does not choose toast then she has a bread roll. If she chooses toast then the probability that she will have jam on it is 0.8. If she has a bread roll then the probability that she will have jam on it is 0.4.

(i) Draw a fully labelled tree diagram to show this information.

(ii) Given that Maria did not have jam for breakfast, find the probability that she had toast.

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Nov 2009 p61 q6
2639

A box contains 4 pears and 7 oranges. Three fruits are taken out at random and eaten. Find the probability that

  1. 2 pears and 1 orange are eaten, in any order,
  2. the third fruit eaten is an orange,
  3. the first fruit eaten was a pear, given that the third fruit eaten is an orange.
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June 2009 p6 q5
2640

At a zoo, rides are offered on elephants, camels and jungle tractors. Ravi has money for only one ride. To decide which ride to choose, he tosses a fair coin twice. If he gets 2 heads he will go on the elephant ride, if he gets 2 tails he will go on the camel ride and if he gets 1 of each he will go on the jungle tractor ride.

(i) Find the probabilities that he goes on each of the three rides.

The probabilities that Ravi is frightened on each of the rides are as follows:

elephant ride \(\frac{6}{10}\), camel ride \(\frac{7}{10}\), jungle tractor ride \(\frac{8}{10}\).

(ii) Draw a fully labelled tree diagram showing the rides that Ravi could take and whether or not he is frightened.

Ravi goes on a ride.

(iii) Find the probability that he is frightened.

(iv) Given that Ravi is not frightened, find the probability that he went on the camel ride.

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Nov 2008 p6 q6
2641

There are three sets of traffic lights on Karinneโ€™s journey to work. The independent probabilities that Karinne has to stop at the first, second and third set of lights are 0.4, 0.8 and 0.3 respectively.

  1. Draw a tree diagram to show this information.
  2. Find the probability that Karinne has to stop at each of the first two sets of lights but does not have to stop at the third set.
  3. Find the probability that Karinne has to stop at exactly two of the three sets of lights.
  4. Find the probability that Karinne has to stop at the first set of lights, given that she has to stop at exactly two sets of lights.
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