Answer
$6$
Work Step by Step
$C(n,r)=\displaystyle \frac{n!}{(n-r)!r!}$.
Factorials:
$n!=n(n-1)!=n(n-1)(n-2)!=...$
$n!=n(n-1)(n-2)\cdots(3)(2)(1)$
$ 0!=1,\qquad$ by definition.
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$C(4,2)=\displaystyle \frac{4!}{(4-2)!2!}=\frac{4\times 3\times 2!}{(2!)2!}$
$=\displaystyle \frac{4\times 3}{2\times 1}$
$=2\times 3=6$