Answer
$$40\sqrt5$$
Work Step by Step
RECALL:
(1) For any real numbers $a \ge 0, b\ge0$, $\sqrt{ab}=\sqrt{a} \cdot \sqrt{b}$
(2) For any real numbers $a \ge 0, b\gt 0$, $\sqrt{\dfrac{a}{b}}=\dfrac{\sqrt{a}}{\sqrt{b}}$
Note that $320=64(5)$.
Thus, using rule (1) above gives:
\begin{align*}
5\sqrt{320}&=5\sqrt{64(5)}\\
&=5\sqrt{64} \cdot \sqrt5\\
&=5(8)\sqrt5\\
&=40\sqrt5
\end{align*}
