а) \(\dfrac{2.8}{16.8}=\dfrac{28}{168}=\dfrac{1}{6}\).
б) \(2\dfrac{1}{4}=\dfrac{9}{4}\), \(1\dfrac{3}{8}=\dfrac{11}{8}\). Тогда \(\dfrac{9}{4}:\dfrac{11}{8}=\dfrac{9}{4}\cdot\dfrac{8}{11}=\dfrac{18}{11}=1\dfrac{7}{11}\).
в) \(3\dfrac{2}{3}=\dfrac{11}{3}\), \(1.21=\dfrac{121}{100}\). Тогда \(\dfrac{121}{100}:\dfrac{11}{3}=\dfrac{121}{100}\cdot\dfrac{3}{11}=\dfrac{33}{100}=0.33\).
Ответ: а) \(\frac{1}{6}\); б) \(1\frac{7}{11}\); в) \(0.33\).