a) $\frac{6}{7}$ + $\frac{5}{7}$ : 5 - $\frac{8}{9}$
= $\frac{6}{7}$ + $\frac{1}{7}$ - $\frac{8}{9}$
= 1 - $\frac{8}{9}$
= $\frac{9}{9}$ - $\frac{8}{9}$
= $\frac{1}{9}$.
b) $\frac{1}{2}$ . ($\frac{2}{9}$ + $\frac{3}{7}$ - $\frac{5}{27}$)
= $\frac{1}{2}$ . ($\frac{14}{63}$ + $\frac{27}{63}$ - $\frac{5}{27}$)
= $\frac{1}{2}$ . ($\frac{41}{63}$ - $\frac{5}{27}$)
= $\frac{1}{2}$ . $\frac{-4}{63}$
= $\frac{-2}{63}$
c) ($\frac{-5}{28}$ + 1,75 + $\frac{8}{35}$) : (-3$\frac{9}{20}$)
= ($\frac{-5}{28}$ + $\frac{7}{4}$ + $\frac{8}{35}$) : $\frac{-69}{20}$
= ($\frac{11}{7}$ + $\frac{8}{35}$) : $\frac{-69}{20}$
= $\frac{9}{5}$ x $\frac{20}{-69}$
= $\frac{-12}{23}$