In how many ways can we choose $n$ objects from $k$ different objects,
if the order of choice does not matter?
.
${k(k-1)\cdots(k-n+1) \over n\cdot(n-1)\cdots2\cdot 1}$
.
${n(n-1)\cdots(n-k+1) \over k\cdot(k-1)\cdots2\cdot 1}$
.