Решение:

1. а) $$rac{2,8}{16,8} = \frac{28}{168} = \frac{1}{6}$$
2. б) $$2\frac{1}{4} : 3\frac{1}{8} = \frac{9}{4} : \frac{25}{8} = \frac{9}{4} \cdot \frac{8}{25} = \frac{9 \cdot 2}{25} = \frac{18}{25}$$
3. в) $$1,21 : 3\frac{2}{3} = \frac{121}{100} : \frac{11}{3} = \frac{121}{100} \cdot \frac{3}{11} = \frac{11 \cdot 3}{100} = \frac{33}{100} = 0,33$$