A train running at 72 km/h crosses a pole in 12 seconds. How much time (in…

When a train crosses a stationary point object such as a pole, signal, or person, the distance it covers equals the train's own length, since only the engine-to-last-coach length needs to pass that point. When it crosses an extended object such as a bridge or platform of length L, the distance covered equals the train's own length plus L, because the front must clear the full extended length before the last coach also clears it. The train's speed stays the same in both crossings, so distance = speed x time applies identically each time.

1. Convert the speed to consistent units: 72 km/h x 5/18 = 20 m/s.

2. Crossing the pole covers only the train's own length: length = speed x time = 20 x 12 = 240 m.

3. Crossing the 360 m bridge covers the train's length plus the bridge's length: total distance = 240 + 360 = 600 m.

4. Time to cross the bridge = total distance / speed = 600 / 20 = 30 seconds.

This can be checked with an equivalent route: the extra distance beyond the pole-crossing is exactly the bridge's own 360 m, which takes 360 / 20 = 18 s at the same speed; adding that to the 12 s already needed to clear the pole gives 12 + 18 = 30 s, the same result.