20) 3 a² 8 / a 12 a³ =

$$\frac{3a^2}{a} \cdot \frac{8}{12a^3} = \frac{3a^2 \cdot 8}{a \cdot 12a^3} = \frac{24a^2}{12a^4} = \frac{2}{a^2}$$