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June 2012 p63 q4
2940
The six faces of a fair die are numbered 1, 1, 1, 2, 3, 3. The score for a throw of the die, denoted by the random variable W, is the number on the top face after the die has landed.
Find the mean and standard deviation of W. [3]
The die is thrown twice and the random variable X is the sum of the two scores. Draw up a probability distribution table for X. [4]
The die is thrown n times. The random variable Y is the number of times that the score is 3. Given that E(Y) = 8, find Var(Y). [3]
Solution
(i) To find the mean of W, calculate the expected value:
Mean = \(\frac{1+1+1+2+3+3}{6} = \frac{11}{6} \approx 1.83\)