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

Answer 1

The train travelled 30km at 30km/hr. It took therefore 30/30=1 hr.
To attain an average of 60km per hour over 120 km, the total time must be 120/60=2 hours.
There is (2-1)=1 hour left to do the remaining 90km. What should be the speed?

Answer 2

120