A particular type of bird lays 1, 2, 3 or 4 eggs in a nest each year. The probability of x eggs is equal to kx, where k is a constant.
- Draw up a probability distribution table, in terms of k, for the number of eggs laid in a year and find the value of k.
- Find the mean and variance of the number of eggs laid in a year by this type of bird.
Solution
(i) The probability distribution table is:
Since the total probability must equal 1, we have:
\(k + 2k + 3k + 4k = 1\)
\(10k = 1\)
Thus, \(k = \frac{1}{10}\).
(ii) The mean \(E(X)\) is calculated as:
\(E(X) = 1 \times \frac{1}{10} + 2 \times \frac{2}{10} + 3 \times \frac{3}{10} + 4 \times \frac{4}{10}\)
\(E(X) = \frac{1}{10} + \frac{4}{10} + \frac{9}{10} + \frac{16}{10} = 3\)
The variance \(\text{Var}(X)\) is calculated as:
\(\text{Var}(X) = \left(1^2 \times \frac{1}{10} + 2^2 \times \frac{2}{10} + 3^2 \times \frac{3}{10} + 4^2 \times \frac{4}{10}\right) - E(X)^2\)
\(\text{Var}(X) = \left(\frac{1}{10} + \frac{8}{10} + \frac{27}{10} + \frac{64}{10}\right) - 3^2\)
\(\text{Var}(X) = \frac{100}{10} - 9 = 1\)
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