Have the answer to physics question but can't understand how they came up with it?

Question

Answer: 4.7 x 10^2 seconds
A train has a mass of 1.5 x 10^7 kg. If the train can exert a constant pull of 7.5 x 10^5 N, How long would it take to increase the speed of train from rest to 85 km/h?

Answer 1

To solve this you need to use the equation F=ma

You can find that the acceleration that can be achieved by the train is 0.05ms^-2.

Next find 85km/h as a speed in m/s. (23.61m/s)

Now divide 23.61 by 0.05 which gives 472. (from V=at)

This is rounds to 4.7 x 10^2 seconds (to 2 significant figures)

Answer 2

F = m*a
V = a*t