A train starts from rest and moves with uniform acceleration for 5 minutes. It then maintains a constant speed for 20 minutes before being brought to rest by a uniform retardation of magnitude twice that of the acceleration. Sketch a speed time graph of the motion and find the time it takes the train to stop. If the train travels 4.5 km while accelerating, show that the acceleration is and find the total distance travelled.

Not sure how to work out how long it takes for the train to stop.2.5 minutes to stop... why?