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A train travelling at 48 kmph crosses another train, having half its length and travelling in opposite direction at 42 kmph, in 12 sec. It also covers a bridge in 45 sec. Find the length of the bridge ?

Correct Answer: 400 mts

Explanation:

Let the length of the 1st train = L mts


Speed of 1st train = 48 kmph


Now the length of the 2nd train = L/2 mts


Speed of 2nd train = 42 kmph


Let the length of the bridge = D mts


Distance = L + L/2 = 3L/2


Relative speed = 48 + 42 = 90 kmph = 90 x 5/18 = 25 m/s(opposite)


Time = 12 sec


=> 3L/2x25 = 12


=> L = 200 mts


Now it covers the bridge in 45 sec


=> distance = D + 200


Time = 45 sec


Speed = 48 x5/18 = 40/3 m/s


=> D + 200/(40/3) = 45


=> D = 600 - 200 = 400 mts


Hence, the length of the bridge = 400 mts.


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