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June 2017 p62 q4
2548
Two identical biased triangular spinners with sides marked 1, 2 and 3 are spun. For each spinner, the probabilities of landing on the sides marked 1, 2 and 3 are \(p\), \(q\) and \(r\) respectively. The score is the sum of the numbers on the sides on which the spinners land. You are given that \(P(\text{score is } 6) = \frac{1}{36}\) and \(P(\text{score is } 5) = \frac{1}{9}\). Find the values of \(p, q\) and \(r\).
Solution
To find \(r\), note that the only way to get a score of 6 is by landing on 3 on both spinners, so \(P(\text{score is } 6) = r^2 = \frac{1}{36}\). Solving for \(r\), we get \(r = \frac{1}{6}\).
For a score of 5, the possible outcomes are (2, 3) and (3, 2). Thus, \(P(\text{score is } 5) = P(2, 3) + P(3, 2) = qr + rq = 2qr = \frac{1}{9}\). Substituting \(r = \frac{1}{6}\), we have: