A train travelling with uniform speed crosses two bridges

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A train travelling with uniform speed crosses two bridges of lengths 300 m and 240 m in 21 s and 18 s respectively. Find the speed of the train?

1. 72 km/h
2. 68 km/h
3. 65 km/h
4. 60 km/h

Correct Option: A

Let length of the train = L
According to the question,
(L + 300)/21 = (L + 240)/18
⇒ (L + 300)/7 = (L + 240)/6
⇒ 6L + 1800 = 7L + 1680
∴ L = 120 m

Taking the length of the 2nd bridge into consideration,
Speed of train = (L+ 240)/18 = (120 + 240)/18 m/s
= (360/18) x (18/5) km/h
= 72 km/h