Browsing as Guest. Progress, bookmarks and attempts are disabled.
Log in to track your work.
June 2011 p62 q7
2945
Judy and Steve play a game using five cards numbered 3, 4, 5, 8, 9. Judy chooses a card at random, looks at the number on it and replaces the card. Then Steve chooses a card at random, looks at the number on it and replaces the card. If their two numbers are equal the score is 0. Otherwise, the smaller number is subtracted from the larger number to give the score.
(i) Show that the probability that the score is 6 is 0.08.
(ii) Draw up a probability distribution table for the score.
(iii) Calculate the mean score.
If the score is 0 they play again. If the score is 4 or more Judy wins. Otherwise Steve wins. They continue playing until one of the players wins.
(iv) Find the probability that Judy wins with the second choice of cards.
(v) Find an expression for the probability that Judy wins with the nth choice of cards.
Solution
(i) The score is 6 if the cards drawn are (3, 9) or (9, 3). The probability of each pair is \(\frac{1}{5} \times \frac{1}{5} = \frac{1}{25}\). Therefore, the probability of a score of 6 is \(2 \times \frac{1}{25} = \frac{2}{25} = 0.08\).
(ii) The probability distribution table is:
x
0
1
2
3
4
5
6
Prob
0.2
0.24
0.08
0.08
0.16
0.16
0.08
(iii) The mean score is calculated as \(\sum x p = 0 \times 0.2 + 1 \times 0.24 + 2 \times 0.08 + 3 \times 0.08 + 4 \times 0.16 + 5 \times 0.16 + 6 \times 0.08 = 2.56\).
(iv) Judy wins if the score is 4, 5, or 6. The probability of these scores is \(0.16 + 0.16 + 0.08 = 0.4\). The probability of a draw (score 0) is 0.2. Therefore, the probability that Judy wins with the second choice is \(0.2 \times 0.4 = 0.08\).
(v) The probability that Judy wins with the \(n\)th choice is \((0.2)^{n-1} \times 0.4\).