Вычислите: a) $$\left(\frac{2}{5} - \frac{1}{4}\right) + \frac{9}{20}$$ б) $$\frac{7}{30} + \left(\frac{3}{5} - \frac{1}{6}\right)$$ в) $$\frac{7}{8} - \left(\frac{1}{9} + \frac{2}{3}\right)$$ г) $$\left(\frac{5}{14} + \frac{9}{10}\right) - \frac{5}{7}$$

a) $$\left(\frac{2}{5} - \frac{1}{4}\right) + \frac{9}{20} = \frac{8}{20} - \frac{5}{20} + \frac{9}{20} = \frac{8-5+9}{20} = \frac{12}{20} = \frac{3}{5}$$ Ответ: $$\frac{3}{5}$$ б) $$\frac{7}{30} + \left(\frac{3}{5} - \frac{1}{6}\right) = \frac{7}{30} + \frac{18}{30} - \frac{5}{30} = \frac{7 + 18 - 5}{30} = \frac{20}{30} = \frac{2}{3}$$ Ответ: $$\frac{2}{3}$$ в) $$\frac{7}{8} - \left(\frac{1}{9} + \frac{2}{3}\right) = \frac{7}{8} - \frac{1}{9} - \frac{2}{3} = \frac{7}{8} - \frac{1}{9} - \frac{6}{9} = \frac{7}{8} - \frac{7}{9} = \frac{63}{72} - \frac{56}{72} = \frac{7}{72}$$ Ответ: $$\frac{7}{72}$$ г) $$\left(\frac{5}{14} + \frac{9}{10}\right) - \frac{5}{7} = \frac{25}{70} + \frac{63}{70} - \frac{50}{70} = \frac{25 + 63 - 50}{70} = \frac{38}{70} = \frac{19}{35}$$ Ответ: $$\frac{19}{35}$$