$\frac{6}{7} + \frac{5}{8} = \frac{48}{56} + \frac{35}{56} = \frac{83}{56}$
$\frac{3}{5} - \frac{3}{8} = \frac{24}{40} - \frac{15}{40} = \frac{9}{40}$
$\frac{1}{4}+\frac{5}{6}=\frac{3}{12}+\frac{10}{12}=\frac{13}{12}$
$\frac{4}{9}-\frac{1}{6}=\frac{8}{18}-\frac{3}{18}=\frac{5}{18}$