On a 60 km track, a train travels the first 30 km with a uniform speed of `30 kmh^(-1)`. How fast must the train travel the next 30 km so as to average `40 kmh^(-1)` for the entire trip ?

Explanation

`V_("avg")=(S_(1)+S_(2))/(t_(1)+t_(2))=(S+S)/(S((1)/(V_(1))+(1)/(V_(2))))=(2V_(1)V_(2))/(V_(1)+V_(2))`
or `40=(2xx30xx v_(2))/(V_(1)+V_(2)) rArr V_(2) =60 kmh^(-1)`