Question
It takes 24 sec for a train travelling at 93 km/h to cross entirely another train half its length travelling in opposite direction at 51 km/h and passes a bridge in 66 sec. What is the length of the bridge in meters?
Solution
Relative speed of the trains = 93 + 51 = 144 km/h = 40 m/s. Let length of the longer train be l meters. So 24 = ${l + l/2}/40$, which gives 24 × 40 = ${3l}/2$, ⇒ l = ${24 × 40 × 2}/3$ = 640 meters. If bridge is L meters, then 66 = ${L + 640}/93$, so L = ${66 × 93 × 5}/18 - 640$ = 1705 - 640 = 1065 meters.
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