Let the normal speed of train = x km/h
time taken to cover 140 km =
If the speed is increased by 42 km/h from its normal speed, journey becomes 3 hours less.
speed = (x+42)km/h
Solving the equation, we get x= 28 or -72. ignore -72 as speed can't be negative.
So the normal speed of train is 28 km/h