\[\boxed{\text{5.411.}}\]
\[\textbf{а)}\ \frac{1}{3} + \frac{5}{6} = \frac{2}{6} + \frac{5}{6} = \frac{7}{6} = 1\frac{1}{6}\ \]
\[\textbf{б)}\ \frac{5}{6} - \frac{1}{2} = \frac{5}{6} - \frac{3}{6} = \frac{2}{6} = \frac{1}{3}\ \]
\[\textbf{в)}\ \frac{9}{10} - \frac{4}{5} = \frac{9}{10} - \frac{8}{10} = \frac{1}{10}\ \]
\[\textbf{г)}\ \frac{5}{6} - \frac{5}{12} = \frac{10}{12} - \frac{5}{12} = \frac{5}{12}\text{\ \ }\]
\[\textbf{д)}\ \frac{11}{18} + \frac{1}{6} = \frac{11}{18} + \frac{3}{18} = \frac{14}{18} = \frac{7}{9}\ \]
\[\textbf{е)}\ \frac{5}{12} + \frac{3}{4} = \frac{5}{12} + \frac{9}{12} = \frac{14}{12} =\]
\[= \frac{7}{6} = 1\frac{1}{6}\ \]
\[\textbf{ж)}\ \frac{4}{5} - \frac{7}{15} = \frac{12}{15} - \frac{7}{15} = \frac{5}{15} = \frac{1}{3}\ \]
\[\textbf{з)}\ \frac{11}{21} + \frac{3}{7} = \frac{11}{21} + \frac{9}{21} = \frac{18}{21} = \frac{6}{7}\text{\ \ }\]