661. a) \(\frac{15a(p-q)}{20b(p-q)}\);

$$\frac{15a(p-q)}{20b(p-q)} = \frac{15a}{20b} \cdot \frac{(p-q)}{(p-q)} = \frac{3a}{4b} \cdot 1 = \frac{3a}{4b}$$ Ответ: \(\frac{3a}{4b}\)