Question

A train travells the first 30km of a 120km track with a uniform speed of 30km/hr. What would be the speed of the train to cover the remaining distance of the track so that its average speed is 60km/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

Answer 2

ANSWER

Average speed=Total distance Total time taken

V

av

=

t

d

ort=

V

av

d

For 120 km,

t=

60 km/hr

120 km

=2 hr ......(i)

For 1st 30 km,

t

1

=

30 km/hr

30 km

=1 hr

For remaining 90 km,

t

2

=t−t

1

=2−1=1 hr

speed=

time

distance

or v

2

=

t

2

d

2

=

1

90

=90 km/hr