П.21 Найдите значение выражения: a) $$\frac{5}{12} : \frac{10}{3} - \frac{5}{6} \cdot \frac{2}{15}$$; б) $$\frac{4}{25} \cdot \frac{5}{16} + \frac{7}{16} \cdot \frac{4}{5}$$; в) $$\frac{9}{8} : \frac{5}{8} : \frac{3}{10}$$

a) $$\frac{5}{12} : \frac{10}{3} - \frac{5}{6} \cdot \frac{2}{15} = \frac{5}{12} \cdot \frac{3}{10} - \frac{5}{6} \cdot \frac{2}{15} = \frac{15}{120} - \frac{10}{90} = \frac{1}{8} - \frac{1}{9} = \frac{9}{72} - \frac{8}{72} = \frac{1}{72}$$ б) $$\frac{4}{25} \cdot \frac{5}{16} + \frac{7}{16} \cdot \frac{4}{5} = \frac{20}{400} + \frac{28}{80} = \frac{1}{20} + \frac{7}{20} = \frac{8}{20} = \frac{2}{5}$$ в) $$\frac{9}{8} : \frac{5}{8} : \frac{3}{10} = \frac{9}{8} \cdot \frac{8}{5} : \frac{3}{10} = \frac{9}{8} \cdot \frac{8}{5} \cdot \frac{10}{3} = \frac{9 \cdot 8 \cdot 10}{8 \cdot 5 \cdot 3} = \frac{90}{15} = 6$$ Ответ: a) $$\frac{1}{72}$$ б) $$\frac{2}{5}$$ в) $$6$$