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