Question

A person travelled from one place to another at an average

speed of 40 kilometres/hour and back to the original place at an

average speed of 50 kilometres/hour. What is his average speed in

kilometres/hour during the entire round­trip ?


it's answer should be 400/9 but i don't know how it's came. please help​

Answer 1

Let the distance between the two places be x

Then for going,

Speed = 40 km/h

Distance = x km

Time (1) = x/40 hrs

Again for returning ,

Distance is same = x km

Speed = 50 km/h

Time (2) = x/50 hr

Avg. speed = total distance/total time

= (x + x) /(x/40 + x/50)

= 2x /{( 5x + 4x)/200}

= 2x/{9x/200}

= 400/9