$$ C(n, r) = \frac{n!}{r!(n-r)!} $$
where:
- n is the total number of items
- r is the number of items to select
- ! denotes the factorial function (the product of all positive integers up to that number)
In this case, n = 20 and r = 3, so the number of different trios that could be selected is:
$$ C(20, 3) = \frac{20!}{3!17!} $$
$$ = \frac{20 \cdot 19 \cdot 18}{1 \cdot 2 \cdot 3} = 1140 $$
Therefore, there are 1140 different trios that could be selected from a chorus of 20 singers.