June 2010 p62 q6
2948
A small farm has 5 ducks and 2 geese. Four of these birds are to be chosen at random. The random variable \(X\) represents the number of geese chosen.
- Draw up the probability distribution of \(X\).
- Show that \(E(X) = \frac{8}{7}\) and calculate \(\text{Var}(X)\).
Solution
(i) The possible values of \(X\) are 0, 1, and 2. The probability distribution is:
| \(x\) | 0 | 1 | 2 |
|---|
| \(P(X = x)\) | \(\frac{1}{7}\) | \(\frac{4}{7}\) | \(\frac{2}{7}\) |
(ii) To find \(E(X)\), use the formula:
\(E(X) = \sum x P(X = x) = 0 \times \frac{1}{7} + 1 \times \frac{4}{7} + 2 \times \frac{2}{7} = \frac{8}{7}\).
To find \(\text{Var}(X)\), use the formula:
\(\text{Var}(X) = E(X^2) - (E(X))^2\).
First, calculate \(E(X^2)\):
\(E(X^2) = 0^2 \times \frac{1}{7} + 1^2 \times \frac{4}{7} + 2^2 \times \frac{2}{7} = \frac{12}{7}\).
Then, \(\text{Var}(X) = \frac{12}{7} - \left(\frac{8}{7}\right)^2 = \frac{12}{7} - \frac{64}{49} = \frac{20}{49}\).
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