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We must choose a 5-member team from 12 girls and 10 boys. How many ways are there to make the choice so that there are no more than 3 boys on the team? The correct answer is (22 choose 5) - (12 choose 1)(10 choose 4) - (10 choose 5). I understand the (22 choose 5) part, but where I am confused at is the other two parts. I do not know how to get those parts.. Can anyone help me understand how to get to the solution? |
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From the total number of ways to form a $5$ member team we subtract the numbers corresponding to the cases when exactly $4$ boys or exactly $5$ boys are chosen to form teams that have no more than $3$ boys. The first case happens when we choose $4$ boys and $1$ girl and the second happens when we choose $5$ boys and $0$ girls. This gives us $\binom{12+10}{5}-\binom{12}{1}\binom{10}{4}-\binom{12}{0}\binom{10}{5}$. |
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That solution proceeds by starting with $\binom{22}5$, the total number of possible $5$-person teams, and subtracting the $\binom{12}1\binom{10}4$ teams that have one girl and four boys and the $\binom{10}5$ teams that have five boys. The problem could also be solved by noting that there are $\binom{12}2\binom{10}3$ teams with two girls and three boys, $\binom{12}3\binom{10}2$ teams with three girls and two boys, $\binom{12}4\binom{10}1$ teams with four girls and one boy, and $\binom{12}5$ teams with five girls, and calculating the sum $$\binom{12}2\binom{10}3+\binom{12}3\binom{10}2+\binom{12}4\binom{10}1+\binom{12}5\;,$$ but it’s pretty clearly more efficient to calculate $$\binom{22}5-\binom{12}1\binom{10}4-\binom{10}5\;.$$ |
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