The train is moving at a speed of 20 m / s. What will be the train speed after braking with an acceleration of 0.25 m / s2 for 20 s?

since the train moves with the same acceleration, i.e. uniformly accelerated (equally slowed down), then the following equality is true:
V1 = V0-a * t = 20-5 = 15m / s

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