A train 180 m long runs at 45 km/h and overtakes a man walking at 9 km/h in the same direction along the track. It passes him completely in how many seconds?

The man is moving the same way, so only the difference of the speeds matters: 45 − 9 = 36 km/$h=36\times 5/18=10$ m/s. The train must cover its own 180 m relative to him, taking 180/10 = 18 s. (Ignoring the man's own speed gives 180/12.5 = 14.4 s, and adding the speeds — right only if he walked towards the train — gives 15 m/s and 12 s.)