A train running at 72 km/h crosses a 160 m long train coming from the opposite direction at 54 km/h in 8 seconds. Running at the same 72 km/h, how many seconds will it take to cross a platform 300 m long?

Head-on the speeds add: 72 + 54 = 126 km/h = 35 m/s, so in 8 seconds the two trains cover 280 m — the sum of their lengths. The first train is therefore 280 − 160 = 120 m long. At 72 km/h = 20 m/s it crosses the platform in (120 + 300)/20 = 21 seconds. (Using only the platform's 300 m gives 15 seconds and forgets that the train must clear its own length as well.)