A bus travels the first one third distance at a speed of 10 km/h. The next one third distance at 20 km/h and the last one third distance at 60 km/h. The average speed of the bus is

Let the total distance traveled is x

Time is taken for first one third $=\frac{x/3}{10}\mathrm{hr}$

$$\begin{gathered} \text{ Time taken for second one third }=\frac{\mathrm{x}/3}{20}\mathrm{hr} \\ \text{ Time taken for third one third }=\frac{\mathrm{x}/3}{60}\mathrm{hr} \\ \text{ Average speed }=\frac{\text{ total dis tan ce }}{\text{ total time }} \\ =\frac{3\mathrm{x}}{\frac{\mathrm{x}}{10}+\frac{\mathrm{x}}{20}+\frac{\mathrm{x}}{60}}=\frac{60\times 3}{10} \\ = 18 \mathrm{km}/\mathrm{h} \end{gathered}$$