Answer
$ 1140$
Work Step by Step
Which CD was chosen first not matter - order does not matter.
We use combinations.
Combinations Formula
The number of combinations possible if $r$ items are taken from $n$ items is
${}_{n}C_{r}=\displaystyle \frac{n!}{(n-r)!r!}$
r=3 CDs are taken from n=20,
${}_{n}C_{r}=\displaystyle \frac{20!}{(17)!3!}=\frac{20\times 19\times 18\times 17!}{(17)!3!}$
... after reducing by $17!$
$=\displaystyle \frac{20\times 19\times(18)}{(3\times 2)\times 1}$
... after reducing by 6
$=20\times 19\times(3)$
$=1140$
