Answer
$20$
Work Step by Step
The number of ways of selecting $n$ elements from $N$ elements is given by the combinations rule:
$$\binom{N}{n} = \dfrac{N!}{n! (N-n)!}$$
The number of ways of selecting 3 elements from 6 elements is given by
$$\binom{6}{3}$$
${\displaystyle{\binom{6}{3}}= \dfrac{6!}{3! \,(6-3)!} =\dfrac{6!}{3! 3!} = \dfrac{6 \cdot 5 \cdot 4 \cdot 3 \cdot 2 \cdot 1 }{(3 \cdot 2 \cdot 1)(3 \cdot 2 \cdot 1)}= \dfrac{6 \cdot 5 \cdot 4}{3 \cdot 2 \cdot 1}}$
${\displaystyle{\binom{6}{3}}= \dfrac{6 \cdot 5 \cdot 4}{3 \cdot 2 \cdot 1}= \dfrac{120}{6} = \boxed{20}}$
