Answer
$\displaystyle \frac{n!}{r!(n-r)!}$
$4$
Work Step by Step
see p.881.
$\displaystyle \left(\begin{array}{l}
n\\
r
\end{array}\right)=\frac{n!}{r!(n-r)!}$
$\displaystyle \left(\begin{array}{l}
4\\
3
\end{array}\right)=\frac{4!}{3!(4-3)!}=\frac{4\times 3\times 2\times 1}{3\times 2\times 1\times 1}=4$
