Team of Four

Coach Martin needs to pick a team for the annual Mathletes competition for this year. There are 36 tryouts for the team, 22 boys and 14 girls. Coach says that he needs a team of 4, consisting of 2 boys and 2 girls.

How many different teams can be formed?


The answer is 21021.

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2 solutions

Stewart Feasby
Oct 15, 2014

This problem can be split into 2 parts, the boys and the girls.

Boys

There are 22 boys and only 2 spots for them. For this we can utilise n C r nCr where we are picking a subset of 2 (r) from a set of 22 (n) objects and, as we're picking a team, we do not care about the order. ( 22 2 ) = 22 ! ( 22 2 ) ! 2 ! = 22 × 21 × . . . × 2 × 1 20 × 19 × . . . × 2 × 1 2 × 1 = 22 × 21 2 = 231 \begin{pmatrix} 22 \\ 2 \end{pmatrix}=\frac { 22! }{ (22-2)!\cdot 2! } =\frac { 22\times 21\times ...\times 2\times 1 }{ 20\times 19\times ...\times 2\times 1\cdot 2\times 1 } =\frac { 22\times 21 }{ 2 } =231

Girls

Now, we can do a similar strategy for the girls, using 14 and 2 as our value of n and r: ( 14 2 ) = 14 ! ( 14 2 ) ! 2 ! = 14 × 13 × . . . × 2 × 1 12 × 11 × . . . × 2 × 1 2 × 1 = 14 × 13 2 = 91 \begin{pmatrix} 14 \\ 2 \end{pmatrix}=\frac { 14! }{ (14-2)!\cdot 2! } =\frac { 14\times 13\times ...\times 2\times 1 }{ 12\times 11\times ...\times 2\times 1\cdot 2\times 1 } =\frac { 14\times 13 }{ 2 } =91

Finally we need to multiply them together: ( 22 2 ) ( 14 2 ) = 231 × 91 = 21021 \begin{pmatrix} 22 \\ 2 \end{pmatrix}\begin{pmatrix} 14 \\ 2 \end{pmatrix}=231\times 91=\boxed { 21021 }

Surajit Biswas
Oct 15, 2014

Out of 22 boys 2 boys could be selected in 22C2 ways and same applied for girls which is 14C2 ways.... So at a total, it could be selected= (22C2 * 14C2) = 21021 ways.

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