Question

a bridge is 5 M long are hundred M long train crosses the bridge at a speed of 30 M per second find the time taken by train to cross it

Answer 1

 
The Total distance covered to cross the bridge for a train = (100+500) m=600 m
Speed of the train = 30 m/s
Time taken by the train to cross the bridge = Total distance/speed=600/30 = 20 second.

Answer 2

Speed of train = 30m/s
Distance covered in passing the bridge
= length of train + length of bridge

Time taken by the train to cross the bridge
= d/s = 105/30 = 21/6 = 3.5 sec

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