Shortcut Trick The extra time taken to cross the platform instead of the pole is directly due to the length of the platform. Extra time = 51 sec − 27 sec = 24 sec. Speed in m/s = 42 × (5/18) = 35/3 m/s. Length of platform = Speed × Extra time = (35/3) × 24 = 35 × 8 = 280 m. ∴ The correct answer is 280 m. Alternate Method Given: Speed of the train = 42 km/h Time to cross electric pole (T_(1)) = 27 sec Time to cross platform (T_(2)) = 51 sec Formula Used: Distance = Speed × Time Speed (m/s) = Speed (km/h) × (5/18) ⇒ Speed in m/s = 42 × (5/18) = 35/3 m/s ⇒ Length of train (L) = Speed × Time to cross pole ⇒ L = (35/3) × 27 = 35 × 9 = 315 m ⇒ Total distance to cross platform = Length of train + Length of platform (P) ⇒ L + P = Speed × Time to cross platform ⇒ 315 + P = (35/3) × 51 ⇒ 315 + P = 35 × 17 ⇒ 315 + P = 595 ⇒ P = 595 − 315 = 280 m ∴ The correct answer is 280 m. Related Concepts: Unit Conversion To convert km/h to m/s, multiply by 5/18. To convert m/s to km/h, multiply by 18/5. Train Crossing a Point Object When a train crosses an electric pole, a tree, or a stationary person, the distance covered is equal to the length of the train. Train Crossing a Large Object When a train crosses a platform, a bridge, or another stationary train, the distance covered is equal to the Sum of the lengths of the train and the object.