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Nov 2018 p62 q6
2907
A fair red spinner has 4 sides, numbered 1, 2, 3, 4. A fair blue spinner has 3 sides, numbered 1, 2, 3. When a spinner is spun, the score is the number on the side on which it lands. The spinners are spun at the same time. The random variable X denotes the score on the red spinner minus the score on the blue spinner.
Draw up the probability distribution table for X.
Find \(\text{Var}(X)\).
Find the probability that \(X\) is equal to 1, given that \(X\) is non-zero.
Solution
(i) To find the probability distribution of \(X\), calculate \(X = R - B\) for each combination of red (R) and blue (B) spinner outcomes. The possible values of \(X\) are -2, -1, 0, 1, 2, 3. The probability distribution table is:
x
-2
-1
0
1
2
3
p
\(\frac{1}{12}\)
\(\frac{2}{12}\)
\(\frac{3}{12}\)
\(\frac{3}{12}\)
\(\frac{2}{12}\)
\(\frac{1}{12}\)
(ii) The variance \(\text{Var}(X)\) is calculated using: