# How do you calculate: 3 over square root of 20?

$3 \cdot \frac{\sqrt{5}}{10}$
Note that $\sqrt{20} = \sqrt{4 \cdot 5} = 2 \sqrt{5}$
$\frac{3}{\sqrt{20}} = \frac{3}{2 \sqrt{5}} = \frac{3 \cdot \sqrt{5}}{10}$