Three coins A, B and C are each thrown once.
- Coins A and B are each biased so that the probability of obtaining a head is \(\frac{2}{3}\).
- Coin C is biased so that the probability of obtaining a head is \(\frac{4}{5}\).
(a) Show that the probability of obtaining exactly 2 heads and 1 tail is \(\frac{4}{9}\).
The random variable \(X\) is the number of heads obtained when the three coins are thrown.
(b) Draw up the probability distribution table for \(X\).
(c) Given that \(\text{E}(X) = \frac{32}{15}\), find \(\text{Var}(X)\).
Solution
(a) To find the probability of obtaining exactly 2 heads and 1 tail, consider the scenarios: HHT, HTH, and THH.
For HHT: \(\frac{2}{3} \times \frac{2}{3} \times \frac{1}{5} = \frac{4}{45}\)
For HTH: \(\frac{2}{3} \times \frac{1}{3} \times \frac{4}{5} = \frac{8}{45}\)
For THH: \(\frac{1}{3} \times \frac{2}{3} \times \frac{4}{5} = \frac{8}{45}\)
Total probability = \(\frac{4}{45} + \frac{8}{45} + \frac{8}{45} = \frac{20}{45} = \frac{4}{9}\)
(b) The probability distribution table for \(X\) is:
| \(x\) | 0 | 1 | 2 | 3 |
| Prob. | \(\frac{1}{45}\) | \(\frac{8}{45}\) | \(\frac{20}{45}\) | \(\frac{16}{45}\) |
(c) Given \(\text{E}(X) = \frac{32}{15}\), find \(\text{Var}(X)\):
\(\text{Var}(X) = 0^2 \times \frac{1}{45} + 1^2 \times \frac{8}{45} + 2^2 \times \frac{20}{45} + 3^2 \times \frac{16}{45} - \left( \frac{32}{15} \right)^2\)
\(= \frac{8}{45} + \frac{80}{45} + \frac{144}{45} - \left( \frac{32}{15} \right)^2\)
\(= \frac{136}{225}\) or 0.604
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