Question

A train travels from city A to city B with a constant speed of 10 m$s-1$ and returns back to city A with a constant speed of 20$s-1$. Find its average speed during its entire journey.

Answer 1

Let the distance between the two cities A and B be x m.
Time taken by the train to travel from A to B =$\frac{x}{10}$ = $t_1$ (say)
Time taken to come back from B to A =$\frac{x}{20}$ = $t_2$ (say)
Average speed = $\frac{Totaldistance}{Totaltime}$ = $\frac{x+x}{t_1+t_2}$ = $\frac{2x}{\frac{x}{10}+\frac{x}{20}}$ = $\frac{40}{3}$ m$s^{-1}$