The speed of a train T is 100 km per hour and the speed of a person P is 4 km per hour. T crosses P in 15 seconds, if P travels along the direction of motion of T. If P travels along the opposite direction of T, then in how much time does T cross P, in seconds, approximately?
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In the same direction the relative speed is 96 km/h, so train length is 96×15/3.6=400 m. In opposite directions relative speed is 104 km/h, giving 400/(104/3.6)≈13.85 s. Hence C.