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Nov 2013 p61 q7
2936
James has a fair coin and a fair tetrahedral die with four faces numbered 1, 2, 3, 4. He tosses the coin once and the die twice. The random variable X is defined as follows.
If the coin shows a head then X is the sum of the scores on the two throws of the die.
If the coin shows a tail then X is the score on the first throw of the die only.
(i) Explain why X = 1 can only be obtained by throwing a tail, and show that P(X = 1) = \(\frac{1}{8}\).
(ii) Show that P(X = 3) = \(\frac{3}{16}\).
(iii) Copy and complete the probability distribution table for X.
x
1
2
3
4
5
6
7
8
P(X = x)
\(\frac{1}{8}\)
\(\frac{3}{16}\)
\(\frac{1}{8}\)
Event Q is ‘James throws a tail’. Event R is ‘the value of X is 7’.
(iv) Determine whether events Q and R are exclusive. Justify your answer.
Solution
(i) If the coin shows a head, the smallest sum of two dice throws is 2 (1+1). Therefore, X = 1 can only occur if the coin shows a tail and the first die shows 1. The probability of a tail is \(\frac{1}{2}\) and the probability of rolling a 1 is \(\frac{1}{4}\). Thus, \(P(X = 1) = \frac{1}{2} \times \frac{1}{4} = \frac{1}{8}\).
(ii) For X = 3, if the coin shows a head, possible outcomes are (1,2) and (2,1), each with probability \(\frac{1}{16}\). If the coin shows a tail, the first die must show 3, with probability \(\frac{1}{8}\). Therefore, \(P(X = 3) = \frac{2}{32} + \frac{1}{8} = \frac{3}{16}\).
(iii) Completing the table:
X
1
2
3
4
5
6
7
8
Prob
\(\frac{1}{8}\)
\(\frac{5}{32}\)
\(\frac{3}{16}\)
\(\frac{7}{32}\)
\(\frac{1}{8}\)
\(\frac{3}{32}\)
\(\frac{1}{16}\)
\(\frac{1}{32}\)
(iv) Events Q and R are exclusive because if a tail is thrown, X cannot be 7. Thus, \(P(Q \cap R) = 0\).