#### Answer

$5\sqrt{5b}$.

#### Work Step by Step

The given expression is
$=\sqrt{125b}$
Factor as square terms.
$=\sqrt{25\cdot 5b}$
Use product property of square roots.
$=\sqrt{25}\cdot \sqrt{5b}$
Use $25=5^2$.
$=\sqrt{5^2}\cdot \sqrt{5b}$
Simplify.
$=5\sqrt{5b}$.