81. Вычислите: a) $4\frac{4}{7} + 12\frac{5}{7}$; б) $9\frac{8}{15} + 6\frac{13}{15}$; в) $5\frac{4}{7} - 1\frac{5}{7}$; г) $4\frac{8}{15} - 2\frac{13}{15}$

а) $4\frac{4}{7} + 12\frac{5}{7} = (4+12) + (\frac{4}{7} + \frac{5}{7}) = 16 + \frac{9}{7} = 16 + 1\frac{2}{7} = 17\frac{2}{7}$ б) $9\frac{8}{15} + 6\frac{13}{15} = (9+6) + (\frac{8}{15} + \frac{13}{15}) = 15 + \frac{21}{15} = 15 + 1\frac{6}{15} = 16\frac{6}{15} = 16\frac{2}{5}$ в) $5\frac{4}{7} - 1\frac{5}{7} = 5\frac{4}{7} - 1\frac{5}{7} = 4\frac{11}{7} - 1\frac{5}{7} = (4-1) + (\frac{11}{7} - \frac{5}{7}) = 3 + \frac{6}{7} = 3\frac{6}{7}$ г) $4\frac{8}{15} - 2\frac{13}{15} = 4\frac{8}{15} - 2\frac{13}{15} = 3\frac{23}{15} - 2\frac{13}{15} = (3-2) + (\frac{23}{15} - \frac{13}{15}) = 1 + \frac{10}{15} = 1\frac{2}{3}$