Applied Maths V. Extra Question

A freight of nuclear waste on its way to Sellafield is travelling at a constant speed of 15m/s. Half a kilometre behind, a passenger train, carrying puppies and orphans, is on the same tracks and travelling in the same direction at 40m/s.
i) What is the minimum deceleration needed to avoid a collision?
ii) If the actual deceleration is 1.5m/s^2, find the closest the two trains are to each other.