Adiesel train left abuja and traveled toward johannesburg. a freight train left one hour later traveling 15 km/h faster in an effort to catch up it. after five hours the freight train finally caught up. what was the diesel train's average speed?

Here is the solution:

Start with the fact that both trains have made the same distance= 's'.

In physics distance is a multiplication of speed of your train = 'v' and period of time in which it was moving= 'T' (this is only true when the speed doesn't change during movement, or we talk about avarage speed like in your task).
Then:

"D" symbol refers do diesel train

Freight train moves with 15[km/h] greater speed and it travels 1 [h] shorter then:


As the distance made by both train is the same you can compare the right parts of both equations:


Diesel train moved 6 hours and freight train 5 hours then:





Here is your The average diesel train speed is 75 km/h