Вычислите: а) (2/5 - 1/4) + 9/20; б) 7/30 + (3/5 - 1/6); в) 7/8 - (1/9 + 2/3); г) (5/14 + 9/20) - 5/7.

Решим задания из упражнения 2.199: а) $(\frac{2}{5} - \frac{1}{4}) + \frac{9}{20}$ 1) $\frac{2}{5} - \frac{1}{4} = \frac{8}{20} - \frac{5}{20} = \frac{3}{20}$ 2) $\frac{3}{20} + \frac{9}{20} = \frac{12}{20} = \frac{3}{5}$ б) $\frac{7}{30} + (\frac{3}{5} - \frac{1}{6})$ 1) $\frac{3}{5} - \frac{1}{6} = \frac{18}{30} - \frac{5}{30} = \frac{13}{30}$ 2) $\frac{7}{30} + \frac{13}{30} = \frac{20}{30} = \frac{2}{3}$ в) $\frac{7}{8} - (\frac{1}{9} + \frac{2}{3})$ 1) $\frac{1}{9} + \frac{2}{3} = \frac{1}{9} + \frac{6}{9} = \frac{7}{9}$ 2) $\frac{7}{8} - \frac{7}{9} = \frac{63}{72} - \frac{56}{72} = \frac{7}{72}$ г) $(\frac{5}{14} + \frac{9}{20}) - \frac{5}{7}$ 1) $\frac{5}{14} + \frac{9}{20} = \frac{50}{140} + \frac{63}{140} = \frac{113}{140}$ 2) $\frac{113}{140} - \frac{5}{7} = \frac{113}{140} - \frac{100}{140} = \frac{13}{140}$