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Nov 2014 p62 q3
3036
(i) Four fair six-sided dice, each with faces marked 1, 2, 3, 4, 5, 6, are thrown. Find the probability that the numbers shown on the four dice add up to 5.
(ii) Four fair six-sided dice, each with faces marked 1, 2, 3, 4, 5, 6, are thrown on 7 occasions. Find the probability that the numbers shown on the four dice add up to 5 on exactly 1 or 2 of the 7 occasions.
Solution
(i) To find the probability that the numbers on four dice add up to 5, consider the combinations: (1, 1, 1, 2), (1, 1, 2, 1), (1, 2, 1, 1), (2, 1, 1, 1). Each die has a probability of \(\frac{1}{6}\) for a specific number. The probability for one combination is \(\frac{1}{6} \times \frac{1}{6} \times \frac{1}{6} \times \frac{1}{6} = \frac{1}{1296}\). There are 4 such combinations, so the total probability is \(4 \times \frac{1}{1296} = \frac{1}{324}\).
(ii) Let \(p = \frac{1}{324}\) be the probability from part (i) and \(q = 1 - p = \frac{323}{324}\). We use the binomial probability formula for exactly 1 or 2 successes in 7 trials: \(P(1,2) = \binom{7}{1} p^1 q^6 + \binom{7}{2} p^2 q^5\). Calculate each term: \(\binom{7}{1} \times \frac{1}{324} \times \left(\frac{323}{324}\right)^6 + \binom{7}{2} \times \left(\frac{1}{324}\right)^2 \times \left(\frac{323}{324}\right)^5 = 0.0214\).