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June 2014 p61 q4
2933
A book club sends 6 paperback and 2 hardback books to Mrs Hunt. She chooses 4 of these books at random to take with her on holiday. The random variable X represents the number of paperback books she chooses.
Show that the probability that she chooses exactly 2 paperback books is \(\frac{3}{14}\).
Draw up the probability distribution table for X.
You are given that \(E(X) = 3\). Find \(\text{Var}(X)\).
Solution
(i) The probability of choosing exactly 2 paperback books is calculated using combinations. The number of ways to choose 2 paperback books from 6 is \(\binom{6}{2}\), and the number of ways to choose 2 hardback books from 2 is \(\binom{2}{2}\). The total number of ways to choose 4 books from 8 is \(\binom{8}{4}\).