Question

A train is moving with an initial velocity of 30m/s the brakesapplied to produce retardation of 1.5 m/s calculate the time when it comes to rest

Answer 1

assuming you meant 1.5 m/s^2, then it will of course take

(30m/s)/(1.5 m/s^2) = 20 s

Think about it. If it loses velocity by 1.5 m/s every second, wit will take 20 s to lose all 30 m/s.

Answer 2

V = Vo + a*t = 0,
30 + (-1.5)t = 0,
t = 20 s.