Browsing as Guest. Progress, bookmarks and attempts are disabled.
Log in to track your work.
Nov 2019 p63 q6
2900
A box contains 3 red balls and 5 white balls. One ball is chosen at random from the box and is not returned to the box. A second ball is now chosen at random from the box.
Find the probability that both balls chosen are red.
Show that the probability that the balls chosen are of different colours is \(\frac{15}{28}\).
Given that the second ball chosen is red, find the probability that the first ball chosen is red.
The random variable \(X\) denotes the number of red balls chosen.
Draw up the probability distribution table for \(X\).
Find \(\text{Var}(X)\).
Solution
(i) The probability that the first ball is red is \(\frac{3}{8}\). After removing one red ball, the probability that the second ball is red is \(\frac{2}{7}\). Thus, the probability that both balls are red is \(\frac{3}{8} \times \frac{2}{7} = \frac{3}{28}\).
(ii) The probability of choosing a red then a white ball is \(\frac{3}{8} \times \frac{5}{7} = \frac{15}{56}\). The probability of choosing a white then a red ball is \(\frac{5}{8} \times \frac{3}{7} = \frac{15}{56}\). Therefore, the probability of different colors is \(\frac{15}{56} + \frac{15}{56} = \frac{30}{56} = \frac{15}{28}\).
(iii) Using conditional probability, \(P(\text{first red} | \text{second red}) = \frac{P(\text{first red and second red})}{P(\text{second red})} = \frac{\frac{3}{28}}{\frac{3}{8} \times \frac{2}{7} + \frac{5}{8} \times \frac{3}{7}} = \frac{\frac{3}{28}}{\frac{21}{56}} = \frac{2}{7}\).
(iv) The probability distribution table for \(X\) is:
\(x\)
0
1
2
\(p\)
\(\frac{10}{28}\)
\(\frac{15}{28}\)
\(\frac{3}{28}\)
(v) The expected value \(E(X) = \frac{30}{56} = \frac{15}{28}\). The variance \(\text{Var}(X) = \frac{30}{56} - \left(\frac{15}{28}\right)^2 = \frac{45}{112}\) or 0.402.