#### Answer

$-10 \sqrt{5}$

#### Work Step by Step

In order to simplify a radical, we consider the factors of the number inside of the radical. If any of these factors are perfect squares, meaning that their square root is a whole number, then we can simplify the radical. We know that 5 and 4 are factors of 20. We know that 4 is a perfect square, so we simplify:
$-5 \sqrt{4} \sqrt{5}=-10 \sqrt{5}$