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June 2020 p53 q4
2895
A fair four-sided spinner has edges numbered 1, 2, 2, 3. A fair three-sided spinner has edges numbered -2, -1, 1. Each spinner is spun and the number on the edge on which it comes to rest is noted. The random variable X is the sum of the two numbers that have been noted.
(a) Draw up the probability distribution table for X.
(b) Find Var(X).
Solution
(a) To find the probability distribution table for X, we first list all possible outcomes of the sums of the numbers from the two spinners:
Four-sided spinner outcomes: 1, 2, 2, 3
Three-sided spinner outcomes: -2, -1, 1
Possible sums (X):
1 + (-2) = -1
1 + (-1) = 0
1 + 1 = 2
2 + (-2) = 0
2 + (-1) = 1
2 + 1 = 3
3 + (-2) = 1
3 + (-1) = 2
3 + 1 = 4
Count the occurrences of each sum and divide by the total number of outcomes (12) to get the probabilities:
\(P(X = -1) = \frac{1}{12}\)
\(P(X = 0) = \frac{3}{12}\)
\(P(X = 1) = \frac{3}{12}\)
\(P(X = 2) = \frac{2}{12}\)
\(P(X = 3) = \frac{2}{12}\)
\(P(X = 4) = \frac{1}{12}\)
(b) To find \(\text{Var}(X)\), we first calculate \(\text{E}(X)\):