A train of length 400 m takes 15 seconds to cross a train of length 300 m traveling at 60 km per hour from the opposite direction along a parallel track. What is the speed of the longer train, in km per hour?

When two trains move in opposite directions, their relative speed is the sum of their individual speeds. The total distance covered to cross each other is 400 + 300 = 700 meters. Since it takes 15 seconds, their relative speed is 700 / 15 = 140 / 3 meters per second. Converting this to kilometers per hour gives (140 / 3) * (18 / 5) = 168 kilometers per hour. The speed of the second train is given as 60 kilometers per hour. Subtracting this from the relative speed yields the speed of the longer train, which is 168 - 60 = 108 kilometers per hour.

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