4/5 : 8/10 : 2/20 =

$$\frac{4}{5} : \frac{8}{10} : \frac{2}{20} = \frac{4}{5} \cdot \frac{10}{8} \cdot \frac{20}{2} = \frac{4}{5} \cdot \frac{5}{4} \cdot 10 = 10$$