Question

On a 120 km track , a train travels the first 30 km at a uniform speed of 30 km/hr.
How fast must the train travel the next 90 km so as to average 60km/hr for the entire trip ?

Answer 1

Then it would be-
Time taken for the first 30km= 30km/30kmh^-1= 1hr
Hence time remaining to cover the remaining 90km=2hrs-1hr= 1hr
Speed during covering of 90km= 90kmh^-1

Answer 2

Av. Velocity = u+v÷t

u= 30 , v

Av. Velocity = 60

60=30+v÷2

60*2=30+v

120-30=v

90=v

90km/he

Hope this helps you..... :-))