A bus travels 54 km in 90 minutes. The speed of the bus is

$$\begin{gathered} \mathrm{Distance} \mathrm{travel}: 54 \mathrm{km} \\ \mathrm{time} \mathrm{taken}: 90 \min =\frac{3}{2}\mathrm{h} \\ \mathrm{Speed}=\frac{\mathrm{Distance}}{\mathrm{Time}} \end{gathered}$$

$$\begin{gathered} \mathrm{Speed}=\frac{54}{{\displaystyle \frac{3}{2}}} \mathrm{km}/\mathrm{h} \\ =54\times \frac{2}{3} \mathrm{km}/\mathrm{h}=36 \mathrm{km}/\mathrm{h} \end{gathered}$$