5/7 : 21/15 : 9/24 =

$$\frac{5}{7} : \frac{21}{15} : \frac{9}{24} = \frac{5}{7} \cdot \frac{15}{21} \cdot \frac{24}{9} = \frac{5}{7} \cdot \frac{5}{7} \cdot \frac{8}{3} = \frac{200}{147}$$