A box contains 10 pens of which 3 are new. A random sample of two pens is taken.
(ii) Construct a probability distribution table for the number of new pens in the sample.
(iii) Calculate the expected number of new pens in the sample.
Solution
To solve part (ii), we need to find the probability distribution for the number of new pens in a sample of two pens.
Let \(X\) be the random variable representing the number of new pens in the sample. \(X\) can take values 0, 1, or 2.
1. Probability of 0 new pens: \(P(X = 0) = \frac{7}{10} \times \frac{6}{9} = \frac{7}{15}\).
2. Probability of 1 new pen: \(P(X = 1) = \left( \frac{3}{10} \times \frac{7}{9} \right) + \left( \frac{7}{10} \times \frac{3}{9} \right) = \frac{7}{15}\).
3. Probability of 2 new pens: \(P(X = 2) = \frac{3}{10} \times \frac{2}{9} = \frac{1}{15}\).
| x | 0 | 1 | 2 |
|---|
| P(X=x) | \(\frac{7}{15}\) | \(\frac{7}{15}\) | \(\frac{1}{15}\) |
For part (iii), calculate the expected value \(E(X)\):
\(E(X) = 0 \times \frac{7}{15} + 1 \times \frac{7}{15} + 2 \times \frac{1}{15} = \frac{7}{15} + \frac{2}{15} = \frac{9}{15} = \frac{3}{5}\).
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