A train $150$ m long is moving at $54$ km/hr. How long will it take to completely cross a platform $250$ m long?

**Step 1 — convert speed.** $54$ km/hr $= 54 \times \dfrac{5}{18} = 15$ m/s. **Step 2 — find the distance the front of the train travels.** To completely cross a platform, the train moves a distance equal to its own length plus the platform's: $150 + 250 = 400$ m. **Step 3 — time.** $\dfrac{400}{15} = 26\tfrac{2}{3}$ s. **Why option A (20 s) is tempting.** It uses only the platform length $250 / 15 \approx 16.7$, then rounds. But the back of the train must clear the platform — the train's own length matters.