Решение:

a) $$\frac{7}{8} \cdot \frac{9}{28} = \frac{7 \cdot 9}{8 \cdot 28} = \frac{7 \cdot 9}{8 \cdot 4 \cdot 7} = \frac{9}{8 \cdot 4} = \frac{9}{32}$$

б) $$\frac{18}{19} : \frac{27}{38} = \frac{18}{19} \cdot \frac{38}{27} = \frac{18 \cdot 38}{19 \cdot 27} = \frac{2 \cdot 9 \cdot 2 \cdot 19}{19 \cdot 3 \cdot 9} = \frac{2 \cdot 2}{3} = \frac{4}{3} = 1\frac{1}{3}$$

в) $$(\frac{2}{3})^4 \cdot (\frac{3}{2})^4 = \frac{2^4}{3^4} \cdot \frac{3^4}{2^4} = \frac{2^4 \cdot 3^4}{3^4 \cdot 2^4} = 1$$

Ответ: a) $$\frac{9}{32}$$; б) $$1\frac{1}{3}$$; в) $$1$$