How to transform a fraction with square root in denominator?

Question

Simple question but my math is a bit rusty.

How does $\frac{20}{\sqrt5} = 4\sqrt5$?

I tried multiplying by $\frac{\sqrt5}{\sqrt5}$ but that's not right.

Answer 1

$$\begin{align} \frac{20}{\sqrt{5}} &= \frac{20}{\sqrt{5}}\cdot\frac{\sqrt{5}}{\sqrt{5}} = \frac{20\sqrt{5}}{\sqrt{5}.\sqrt{5}} \\ &= \frac{20\sqrt{5}}{5} = 4\sqrt{5} \end{align}$$

Answer 2

Actually, multiplying the numerator and denominator by $\sqrt 5$ is spot on. Perhaps you made an arithmetic error? Let's try it again:

$$\frac{20}{\sqrt 5} \times \frac {\sqrt 5}{\sqrt 5} = \frac {20\sqrt 5}{5} = 4\sqrt 5$$