[tex]\bf _nC_r\iff \left(
\begin{array}{llll}
n\\
r
\end{array} \right)\iff \cfrac{n!}{r!(n-r)!}\\\\
-----------------------------\\\\
\left(
\begin{array}{llll}
40\\
4
\end{array} \right)\implies \cfrac{40!}{4!(40-4)!}\implies
\begin{array}{llll}
\cfrac{4!}{4!\cdot 36!}\\
\qquad \uparrow \\
\qquad b
\end{array}[/tex]
