Решение:

1. a) $$\frac{3}{8}:\frac{2}{3} = \frac{3}{8} \cdot \frac{3}{2} = \frac{3 \cdot 3}{8 \cdot 2} = \frac{9}{16}$$
2. б) $$\frac{4}{7}:\frac{4}{9} = \frac{4}{7} \cdot \frac{9}{4} = \frac{4 \cdot 9}{7 \cdot 4} = \frac{9}{7} = 1\frac{2}{7}$$
3. в) $$11\frac{2}{5}:3\frac{4}{5} = \frac{11 \cdot 5 + 2}{5} : \frac{3 \cdot 5 + 4}{5} = \frac{57}{5} : \frac{19}{5} = \frac{57}{5} \cdot \frac{5}{19} = \frac{57 \cdot 5}{5 \cdot 19} = \frac{57}{19} = 3$$
4. г) $$\frac{16}{17}:8 = \frac{16}{17} \cdot \frac{1}{8} = \frac{16 \cdot 1}{17 \cdot 8} = \frac{2}{17}$$
5. д) $$12:\frac{6}{7} = 12 \cdot \frac{7}{6} = \frac{12 \cdot 7}{6} = \frac{2 \cdot 7}{1} = 14$$