Question

it is noticed that exactly half a minute elapses between the time when the engine of a train 50m long enters a tunnel of 500mlong and the time when the last carriage of the train leaves the tunnel.find at how many km per hour the train is traveling.
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Answer 1

Assume this means that the REAR of the train emerges from the tunnel 1 minute after the FRONT of the train enters the tunnel - that’s the only way I can think of that the question makes sense.

The front of the train travels 72km/hr * 1 hr / 60 min = 1.2 km in one minute. So the train must be 1200m - 500m = 700 meters long if the rear of the train is only 500 meters further down the track than the front was a minute earlier.