Question

A bus travels the first one-third​ distance at a speed of 10$\frac{km}{h}$, the next one-third​ at 20$\frac{km}{h}$ and the last one-third at 60$\frac{km}{h}$. The average speed of the bus is

• A. 9 km/h
• B. 16 km/h
• C. 18 km/h
• D. 48 km/h

Answer 1

The correct option is C. 18 km/h

Let the distance be 3x.

We know that,

Time=DistanceSpeedTime=\frac{Distance}{Speed}Time=SpeedDistance​
Time taken to travel first one third distance, $t_1$ = $\frac{x}{10}$hours
Time taken to travel second one third distance, $t_2$ = $\frac{x}{20}$hours
Time taken to travel third one third distance, $t_3$ = $\frac{x}{60}$hours
Total time taken = $\frac{x}{10}$hours + $\frac{x}{20}$hours + $\frac{x}{60}$hours = $\frac{x}{6}$hours
Average speed = $\frac{Totaldistance}{Totaltime}$
Average speed = $\frac{3x}{\left(\frac{x}{6}\right)}$=18$\frac{km}{h}$.