Factor $20=2^{2}\times 5$. Rewrite the square root of the product $\sqrt{2^{2}\times 5}$ as the product of square roots $\sqrt{2^{2}}\sqrt{5}$. Take the square root of $2^{2}$.
$$\frac{2\sqrt{5}}{2\sqrt{5}}$$
Cancel out $2\sqrt{5}$ in both numerator and denominator.
