40/50 : 20/5 : 3/4 =

$$\frac{40}{50} : \frac{20}{5} : \frac{3}{4} = \frac{40}{50} \cdot \frac{5}{20} \cdot \frac{4}{3} = \frac{4}{5} \cdot \frac{1}{4} \cdot \frac{4}{3} = \frac{4 \cdot 1 \cdot 4}{5 \cdot 4 \cdot 3} = \frac{16}{60} = \frac{4}{15}$$