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 $2^{2}$.
$$2\sqrt{5}+\sqrt{5}$$
Combine $2\sqrt{5}$ and $\sqrt{5}$ to get $3\sqrt{5}$.
