Question

On a 120km track, a train travels the first 30 km at a uniform speed of 30km/hr. How fast must the train travel the next remaining distance of track
so as to average 60 km/hr for the entire trip?

Answer 1

Speed of the train in the first 30 km, v₁ = 30 km/h
Speed of the train in the next 90 km, v₂ (to be determined)
Average speed = 60 km/h = (v1 + v₂)/2 
                                          = (30 km/h + v₂)/2 

                                        ⇒ v₂ = 90 km/h