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2015-06-18T20:56:23+05:30

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Length of the train = L meters.
Distance travelled by the train to cross the bridge is the length of the bridge plus length of the train.

Speed of the train = distance / time interval
           = (300 + L) m / 15 sec  =  (450 + L) / 21 sec
   =>    21 (300 + L)  = 15 (450 + L)
   =>    2100 + 7 L = 2250 + 5 L
   =>    2 L = 150
   =>  L = 75 meters.

Speed of the train = (300 + 75) meters / 15 sec = 25 m/sec
                 = 25 * 18/5 = 90 km/hour

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