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?

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 mTaking 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