This is a d = rt problem, as you know. Distance equals rate (speed) times time.
If you think about this problem, you will realize that the distance traveled by the two vehicles must be the same, since they start at the same point, and the problem ends when the diesel catches up.
Let rt = the train's speed
Let tt = t he train's time of travel
Let rd = the diesel's speed
Let tt = the diesel's time
Since d is the same for both,
rttt = rdtd (Transitive property, or two things equal to the same thing are equal to each other.)
Now substitute known values. The diesel travels for 4 hours at 45 km/hr (given). The train starts 14 hours earlier, so travels a total of 18 hours. We now have three of the four variables evaluated and can solve for rt.
rt x 18 = 45 x 4
18 rt = 180 (simplifying
rt = 10 (dividing both sides by 18)