A train moves from rest to a speed of 25 m/s in 20 seconds. What is its acceleration?

Hello!
Acceleration is the measure of the changing speed of a body over time, in the same manner as speed is the measure of the changing position of a body.
If we know the speed V_1 for some initial time moment t_1 and the speed V_2 for some other moment t_2, and if we know also that the change of speed was uniform, then this change is the (uniform) acceleration
a = (V_2 - V_1)/(t_2 - t_1).
If we don't know that the change was uniform, then this formula gives us the average acceleration. Note that only a difference between times is required, so we may set a reference point for time anywhere.
For this problem, we know t_2-t_1 = 20 s, V_1 = 0 (moves from rest) and V_2 = 25 m/s, so the acceleration, uniform or average, is  25/20 = 1.25 (m/s^2). This is the answer.