\(\sqrt{20} = \sqrt{4} ⋅ \sqrt{5} = 2 \sqrt{5} \text{.}\)

\(-6 \sqrt{32} = -6 ⋅ \sqrt{16} ⋅ \sqrt{2} = -6 ⋅ 4 ⋅ \sqrt{2} = -24 \sqrt{2} \text{.}\)

\(\sqrt{\frac{49}{81}} = {\sqrt{49} \over \sqrt{81}} = \frac{7}{9} \text{.}\)

\({42 \sqrt{160} \over 7 \sqrt{8}} = {42 \over 7} ⋅ {\sqrt{160} \over \sqrt{8}} = 6 \sqrt{20} = 6 ⋅ \sqrt{4} ⋅ \sqrt{5} = 6 ⋅ 2 ⋅ \sqrt{5} = 12 \sqrt{5}\)

\(\sqrt{1\frac{39}{49}} = \sqrt{\frac{88}{49}} = {\sqrt{88} \over \sqrt{49}} = {\sqrt{88} \over 7} = \frac{1}{7} \sqrt{88} = \frac{1}{7} ⋅ 2 ⋅ \sqrt{22} = \frac{2}{7} \sqrt{22} \text{.}\)