To answer the problem above, we first find the velocity of the train that is travelling. It is calculated as follows:
Velocity = 42 km / 15 min = 2.8 km / min
The distance traveled after 3 hours at this speed is calculated by multiplying the velocity and the number of hours.
Distance = 2.8 km / min ( 3 hours ) ( 60 min / 1 hour ) = 504 km