a)

$\frac{3}{4}:\frac{3}{7}=\frac{{\sout{3}}^1}{4}\cdot \frac{7}{{\sout{3}}^1}=\frac{7}{4}=1\frac{3}{4}$

$\frac{2}{5}:\frac{3}{5}=\frac{2}{{\sout{5}}^1}\cdot \frac{{\sout{5}}^1}{3}=\frac{2}{3}$  

b)

$\frac{4}{5}:\frac{7}{20}=\frac{4}{{\sout{5}}^1}\cdot \frac{{\sout{20}}^4}{7}=$   $\frac{16}{7}=2\frac{2}{7}$