A teain tramels from city `A` to city `B` with constant speed of `10 m s^(-1)` and returns back to city `A` with a constant spiid of `20 m s^(-1)`. Finde its average speed during its entire journey.

Explanation

Let the distance between the two cities `A` and `B` be `x` m.
Time taken by the train to travel from `A` to `B=(x)/(10)=t_(1)` (say)
Time taken to come back from `B` to `A =(x)/(10)=t_(2)` (say)
Average speed `=(Total distance)/(Total time) (x+x)/(t_(1)+t_(2))=(2x)/((x)/(10)+(x)/(10))=(40)/(3) m s^(-1)`