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June 2012 p61 q7
2863
A group of 9 people consists of 2 boys, 3 girls and 4 adults. In how many ways can a team of 4 be chosen if
both boys are in the team,
the adults are either all in the team or all not in the team,
at least 2 girls are in the team?
Solution
(i) If both boys are in the team, we need to choose 2 more people from the remaining 7 people (3 girls and 4 adults). The number of ways to choose 2 people from 7 is given by the combination formula: \(\binom{7}{2} = 21\).
(ii) If the adults are either all in the team or all not in the team, we consider two cases:
All adults in the team: This is not possible since the team size is 4 and there are 4 adults.
All adults not in the team: We choose all 4 members from the 5 non-adults (2 boys and 3 girls). The number of ways is \(\binom{5}{4} = 5\).
Thus, the total number of ways is \(1 + 5 = 6\).
(iii) If at least 2 girls are in the team, we consider two cases:
2 girls in the team: Choose 2 girls from 3 and 2 others from the remaining 6 people (2 boys and 4 adults). The number of ways is \(\binom{3}{2} \times \binom{6}{2} = 3 \times 15 = 45\).
3 girls in the team: Choose 3 girls from 3 and 1 other from the remaining 6 people. The number of ways is \(\binom{3}{3} \times \binom{6}{1} = 1 \times 6 = 6\).
Thus, the total number of ways is \(45 + 6 = 51\).