31. 16m³(a+3)2 20m²(a+3)

$$\frac{16m^3(a+3)^2}{20m^4(a+3)} = \frac{4 \cdot 4m^3(a+3)^2}{5 \cdot 4m^4(a+3)} = \frac{4(a+3)}{5m}$$ Ответ: $$\frac{4(a+3)}{5m}$$