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Nov 2022 p52 q5
2931
Eric has three coins. One of the coins is fair. The other two coins are each biased so that the probability of obtaining a head on any throw is \(\frac{1}{4}\), independently of all other throws. Eric throws all three coins at the same time.
Events \(A\) and \(B\) are defined as follows.
\(A\): all three coins show the same result
\(B\): at least one of the biased coins shows a head
(a) Show that \(P(B) = \frac{7}{16}\).
(b) Find \(P(A \mid B)\).
The random variable \(X\) is the number of heads obtained when Eric throws the three coins.
(c) Draw up the probability distribution table for \(X\).
Solution
(a) To find \(P(B)\), consider the scenarios where at least one biased coin shows a head. The probability that a biased coin shows a tail is \(\frac{3}{4}\). The probability that both biased coins show tails is \(\left(\frac{3}{4}\right)^2 = \frac{9}{16}\). Therefore, \(P(B) = 1 - \frac{9}{16} = \frac{7}{16}\).
(b) To find \(P(A \mid B)\), use the formula \(P(A \mid B) = \frac{P(A \cap B)}{P(B)}\). The event \(A \cap B\) occurs when all coins show heads or all show tails. The probability of all heads is \(\frac{1}{2} \times \frac{1}{4} \times \frac{1}{4} = \frac{1}{32}\). The probability of all tails is \(\frac{1}{2} \times \frac{3}{4} \times \frac{3}{4} = \frac{9}{32}\). Thus, \(P(A \cap B) = \frac{1}{32}\). Therefore, \(P(A \mid B) = \frac{\frac{1}{32}}{\frac{7}{16}} = \frac{1}{14}\).
(c) The probability distribution for \(X\) is calculated as follows: