Answer
15
Work Step by Step
$\sqrt5\times\sqrt{45}\longrightarrow$ factor out the perfect square from 45
=$\sqrt5\times\sqrt{9\times5}\longrightarrow$ multiplication property of square roots
=$\sqrt5\times\sqrt9\times\sqrt5\longrightarrow$ simplify
=$3\times\sqrt5\times\sqrt5\longrightarrow$ simplify
=$3\times5\longrightarrow$ multiply
=$15$
