\(\sqrt{28} = \sqrt{4} ⋅ \sqrt{7} = 2 \sqrt{7} \text{.}\)

\(6 \sqrt{700} = 6 ⋅ \sqrt{100} ⋅ \sqrt{7} = 6 ⋅ 10 ⋅ \sqrt{7} = 60 \sqrt{7} \text{.}\)

\(\sqrt{\frac{4}{81}} = {\sqrt{4} \over \sqrt{81}} = \frac{2}{9} \text{.}\)

\({15 \sqrt{160} \over 5 \sqrt{8}} = {15 \over 5} ⋅ {\sqrt{160} \over \sqrt{8}} = 3 \sqrt{20} = 3 ⋅ \sqrt{4} ⋅ \sqrt{5} = 3 ⋅ 2 ⋅ \sqrt{5} = 6 \sqrt{5}\)

\(\sqrt{\frac{57}{100}} = {\sqrt{57} \over \sqrt{100}} = {\sqrt{57} \over 10} = \frac{1}{10} \sqrt{57} \text{.}\)