\[\boxed{\text{5.542.}}\]
\[\textbf{а)}\ \frac{3}{8}\ :\frac{3}{4} \cdot \frac{2}{3} = \frac{3}{8} \cdot \frac{4}{3} \cdot \frac{2}{3} = \frac{1}{3}\]
\[\textbf{б)}\ \frac{10}{7} \cdot \frac{3}{4}\ :\frac{5}{14} = \frac{10}{7} \cdot \frac{3}{4} \cdot \frac{14}{5} =\]
\[= \frac{1 \cdot 3 \cdot 2}{1 \cdot 2 \cdot 1} = 3\]
\[\textbf{в)}\ \frac{9}{8}\ :\frac{5}{8}\ :\frac{3}{10} = \frac{9}{8} \cdot \frac{8}{5} \cdot \frac{10}{3} =\]
\[= \frac{3 \cdot 1 \cdot 2}{1 \cdot 1 \cdot 1} = 6\]