$\text{a)}\frac{20}{21}:5=\frac{{\sout{20}}^4}{21}\cdot \frac{1}{{\sout{5}}^1}=\frac{4}{21}$

$\text{b)}\frac{14}{39}:\frac{7}{13}=\frac{14}{{\sout{39}}^3}\cdot \frac{{\sout{13}}^1}{7}=\frac{{\sout{14}}^2}{3}\cdot \frac{1}{{\sout{7}}^1}=\frac{2}{3}$

$\text{c)}1\frac{1}{5}:\frac{4}{5}=\frac{6}{{\sout{5}}^1}\cdot \frac{{\sout{5}}^1}{4}=\frac{6}{4}=\frac{3}{2}=1\frac{1}{2}$