Q: A goods train departs from a station at a predetermined time and speed. After many hours, an express train departs from the same station and travels in the same direction at a constant speed of 90 kmph. In 4 hours, this train catches up with the freight train. Calculate the speed of the freight train.  

The speed of the goods train is given as x kmph.

The distance traveled by the goods train in 10 hours is given as 10x km.

The distance traveled by an express train in 4 hours is given as 90 km.

We can set up an equation to represent the relationship between the distance traveled by each train and the time taken to travel that distance. In this case, 

the equation is (distance traveled by goods train in 10 hours) = (distance traveled by express train in 4 hours).

We can solve for the speed of the goods train by rearranging the equation to isolate the speed. 

The equation given is (distance traveled by goods train in 10 hours) = (distance traveled by express train in 4 hours). 

We can solve for x by dividing both sides of the equation by 10 and 4, respectively, 

which gives us (x kmph) = (90 km / 4 hours). 

Solving for x gives us x = 36 kmph.