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Question

In how many ways can we select a chairperson, vice-chairperson, secretary, and treasurer from a group of 12 persons?

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We are interested in the number of 4 people from a set with 12 persons:

n=12r=4\begin{align*} n&=12 \\ r&=4 \end{align*}

The order in which we select the 4 people matters (as a different order of the people results in different chairperson, vice-chairperson, secretary and treasurer), thus we need to use the definition of a permutation\textbf{permutation}.

A r-permutation\textbf{r-permutation} of a set of elements is an ordering of the rr of the elements in the set in a row. The number of rr-permutations of a set of nn distinct objects is P(n,r)=n!(nr)!P(n,r)=\frac{n!}{(n-r)!}.

P(12,4)=12!(114)!=12!8!=12111098765432187654321=1211109=11,880\begin{align*} P(12,4)&=\frac{12!}{(11-4)!} \\ &=\frac{12!}{8!} \\ &=\frac{12\cdot 11\cdot 10\cdot 9\cdot 8\cdot 7\cdot 6\cdot 5\cdot 4\cdot 3\cdot 2\cdot 1}{8\cdot 7\cdot 6\cdot 5\cdot 4\cdot 3\cdot 2\cdot 1} \\ &=12\cdot 11\cdot 10\cdot 9 \\ &=11,880 \end{align*}

Thus there are 11,880 ways to select a chairperson, vice-chairperson, secretary, and treasurer from a group of 12 persons.

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