6/7 : 3/9 : 28/30 =

$$\frac{6}{7} : \frac{3}{9} : \frac{28}{30} = \frac{6}{7} \cdot \frac{9}{3} \cdot \frac{30}{28} = \frac{6}{7} \cdot 3 \cdot \frac{15}{14} = \frac{6 \cdot 3 \cdot 15}{7 \cdot 14} = \frac{270}{98} = \frac{135}{49}$$