Решение:
а) $\frac{3}{8} - \frac{2}{7} + \frac{5}{8} - \frac{5}{7} = (\frac{3}{8} + \frac{5}{8}) - (\frac{2}{7} - \frac{5}{7}) = \frac{8}{8} - \frac{7}{7} = 1 - 1 = 0$;
б) $\frac{11}{14} - \frac{7}{10} - \frac{21}{100} - \frac{13}{14} = (\frac{11}{14} - \frac{13}{14}) + (-\frac{7}{10} - \frac{21}{100}) = -\frac{2}{14} + (-\frac{70}{100} - \frac{21}{100}) = -\frac{2}{14} -\frac{91}{100} = -\frac{1}{7} - \frac{91}{100} = $ $\frac{-100 - 637}{700} = \frac{-737}{700} = -1\frac{37}{700}$;
в) $-\frac{12}{19} - \frac{15}{26} + \frac{3}{19} + \frac{9}{19} = (-\frac{12}{19} + \frac{3}{19} + \frac{9}{19}) - \frac{15}{26} = 0 - \frac{15}{26} = -\frac{15}{26}$;
г) $\frac{2}{7} - \frac{5}{9} - \frac{4}{9} - \frac{4}{7} = (\frac{2}{7} - \frac{4}{7}) + (-\frac{5}{9} - \frac{4}{9}) = -\frac{2}{7} - \frac{9}{9} = -\frac{2}{7} - 1 = -\frac{2}{7} - \frac{7}{7} = -\frac{9}{7} = -1\frac{2}{7}$.
Ответ:
а) $0$;
б) $-1\frac{37}{700}$;
в) $-\frac{15}{26}$;
г) $-1\frac{2}{7}$.