\[\boxed{\text{П.20.}}\]
\[\textbf{а)}\ \frac{16}{27} \cdot \frac{9}{4} = \frac{4}{3} = 1\frac{1}{3}\]
\[\textbf{б)}\ \frac{42}{5} \cdot \frac{55}{7} = 6 \cdot 11 = 66\]
\[\textbf{в)}\ \frac{7}{12}\ :\frac{3}{12} = \frac{7}{12} \cdot \frac{12}{3} = \frac{7}{3} = 2\frac{1}{3}\]
\[\textbf{г)}\ \ \frac{14}{55}\ :\frac{7}{22} = \frac{14}{55} \cdot \frac{22}{7} = \frac{4}{5}\]