#### Answer

$2 \sqrt{10}$

#### 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 4 and 10 are factors of 40. We know that 4 is a perfect square, so we simplify:
$ \sqrt{4} \sqrt{10}=2 \sqrt{10}$