$\frac{2}{9}х + \frac{4}{9}х = (\frac{2}{9} + \frac{4}{9})х = \frac{6}{9}х = \frac{2}{3}х$

$\frac{5}{7}а - \frac{9}{14}а = (\frac{5}{7} - \frac{9}{14})а = (\frac{10}{14} - \frac{9}{14})а = \frac{1}{14}а$

$\frac{7}{12}m - \frac{5}{12}m = (\frac{7}{12} - \frac{5}{12})m = \frac{2}{12}m = \frac{1}{6}m$

$\frac{5}{6}b - \frac{3}{4}b = (\frac{5}{6} - \frac{3}{4})b = (\frac{10}{12} - \frac{9}{12})b = \frac{1}{12}b$

$3\frac{1}{6}z + \frac{2}{3}z = (3\frac{1}{6} + \frac{2}{3})z = (3\frac{1}{6} + \frac{4}{6})z = 3\frac{5}{6}z$

$2\frac{3}{4}t - 1\frac{7}{8}t = (2\frac{3}{4} - 1\frac{7}{8})t = (2\frac{6}{8} - 1\frac{7}{8})t = (1\frac{14}{8} - 1\frac{7}{8})t = 1\frac{7}{8}t$

$\frac{5}{18}х + (\frac{5}{12}х - \frac{1}{4}х) = \frac{5}{18}х + (\frac{5}{12} - \frac{1}{4})х = \frac{5}{18}х + (\frac{5}{12} - \frac{3}{12})х = \frac{5}{18}х + \frac{2}{12}х = \frac{5}{18}х + \frac{1}{6}х = \frac{5}{18}х + \frac{3}{18}х = (\frac{5}{18} + \frac{3}{18})х = \frac{8}{18}х = \frac{4}{9}х$

$\frac{11}{18}n - (\frac{5}{18}n + \frac{1}{6}n) = \frac{11}{18}n - (\frac{5}{18} + \frac{3}{18})n = \frac{11}{18}n - \frac{8}{18}n = (\frac{11}{18} - \frac{8}{18})n = \frac{3}{18}n = \frac{1}{6}n$

$\frac{2}{3}c + \frac{1}{9}c - \frac{7}{9}c = (\frac{6}{9} + \frac{1}{9} - \frac{7}{9})c = 0с = 0$

$k - \frac{1}{7}k = (\frac{7}{7} - \frac{1}{7})k = \frac{6}{7}k$

$\frac{3}{11}y + \frac{8}{11}y = (\frac{3}{11} + \frac{8}{11})y = \frac{11}{11}y = 1y = y$

$\frac{3}{5}b + b = (\frac{3}{5} + 1)b = 1\frac{3}{5}b$