Exam-Style Problems

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June 2013 p62 q7
2627

Susan has a bag of sweets containing 7 chocolates and 5 toffees. Ahmad has a bag of sweets containing 3 chocolates, 4 toffees and 2 boiled sweets. A sweet is taken at random from Susan’s bag and put in Ahmad’s bag. A sweet is then taken at random from Ahmad’s bag.

(i) Find the probability that the two sweets taken are a toffee from Susan’s bag and a boiled sweet from Ahmad’s bag.

(ii) Given that the sweet taken from Ahmad’s bag is a chocolate, find the probability that the sweet taken from Susan’s bag was also a chocolate.

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June 2013 p61 q7
2628

Box A contains 8 white balls and 2 yellow balls. Box B contains 5 white balls and x yellow balls. A ball is chosen at random from box A and placed in box B. A ball is then chosen at random from box B. The tree diagram below shows the possibilities for the colours of the balls chosen.

(i) Justify the probability \(\frac{x}{x+6}\) on the tree diagram.

(ii) Copy and complete the tree diagram.

(iii) If the ball chosen from box A is white then the probability that the ball chosen from box B is also white is \(\frac{1}{3}\). Show that the value of \(x\) is 12.

(iv) Given that the ball chosen from box B is yellow, find the conditional probability that the ball chosen from box A was yellow.

problem image 2628
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Nov 2012 p62 q1
2629

Fabio drinks coffee each morning. He chooses Americano, Cappuccino or Latte with probabilities 0.5, 0.3 and 0.2 respectively. If he chooses Americano he either drinks it immediately with probability 0.8, or leaves it to drink later. If he chooses Cappuccino he either drinks it immediately with probability 0.6, or leaves it to drink later. If he chooses Latte he either drinks it immediately with probability 0.1, or leaves it to drink later.

(i) Find the probability that Fabio chooses Americano and leaves it to drink later.

(ii) Fabio drinks his coffee immediately. Find the probability that he chose Latte.

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Nov 2012 p61 q6
2630

Ana meets her friends once every day. For each day the probability that she is early is 0.05 and the probability that she is late is 0.75. Otherwise she is on time.

If she is early there is a probability of 0.7 that she will eat a banana. If she is late she does not eat a banana. If she is on time there is a probability of 0.4 that she will eat a banana. Given that for one particular meeting with friends she does not eat a banana, find the probability that she is on time.

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June 2012 p61 q2
2631

Maria has 3 pre-set stations on her radio. When she switches her radio on, there is a probability of 0.3 that it will be set to station 1, a probability of 0.45 that it will be set to station 2 and a probability of 0.25 that it will be set to station 3. On station 1 the probability that the presenter is male is 0.1, on station 2 the probability that the presenter is male is 0.85 and on station 3 the probability that the presenter is male is \(p\). When Maria switches on the radio, the probability that it is set to station 3 and the presenter is male is 0.075.

  1. Show that the value of \(p\) is 0.3.
  2. Given that Maria switches on and hears a male presenter, find the probability that the radio was set to station 2.
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