# How to write a combinations formula for this?

I have 8 distinct elements. Each set has 4 pairs from the 8 elements above. How many such distinct sets are possible?

e.g. 8 elements - 1,2,3,4,5,6,7,8 example set - 1,2;3,4;5,6;7,8 the ordering of elements in a pair is not important

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–  Byron Schmuland Jun 7 '12 at 12:23

There are $7$ ways to choose the number to be paired with $1$. Once that pairing has been made, there are $6$ elements left, so there are $5$ ways to decide which gets paired with the smallest of the remaining numbers. After that $4$ elements remain, and there are $3$ ways to decide which gets paired with the smallest of them. The last two elements must form the fourth pair, so the total number of sets of four pairs is $7\cdot5\cdot3=105$.
Added: Alternatively, you can argue as follows. There are $\binom82$ ways to choose a pair, $\binom62$ ways to choose a pair from the remaining $6$ elements, and $\binom42$ ways to choose a pair from the $4$ elements that still remain; the last two elements will of course be the fourth pair. That’s $$\binom82\binom62\binom42=\frac{8!}{2!6!}\cdot\frac{6!}{2!4!}\cdot\frac{4!}{2!2!}=\frac{8!}{2!2!2!2!}=\frac{40320}{16}=2520\;,$$ but it takes into account the order in which the pairs are chosen. Since a given set of $4$ pairs can be chosen in $4!=24$ different orders, we have to divide by $24$ to get $\dfrac{2520}{24}=105$.
$$\frac{8!}{2!2!2!2!4!}=\frac{8\cdot7\cdot6\cdot5\cdot4\cdot3\cdot2}{2^7\cdot3}=7\cdot5\cdot3\;.$$