Question

A body travels one third of the distance at speed 10km/h, next one third at speed 20km/h and the remaining at speed of 40km/h. What is the average speed for entire journey?

• A. $\frac{120}{7}km/hr$
• B. $\frac{80}{7}km/hr$
• C. $\frac{40}{14}km/hr$
• D. $\frac{160}{14}km/hr$

Answer 1

The correct option is A

$\frac{120}{7}km/hr$

We know
Average speed =$\frac{totaldistancecovered}{totaltimetaken}$
Let the total distance be 3d
Time taken to cover first one third distance =$t_1=\frac{d}{10}hr$
Time taken to cover second one third distance =$t_2=\frac{d}{20}hr$
Time taken to cover last one third distance =$t_3=\frac{d}{40}hr$
Average speed =$\frac{3d}{\frac{d}{10}+\frac{d}{20}+\frac{d}{40}}$
On solving
Average speed = $\frac{120}{7}km/hr$