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?

When P moves in same direction as T, relative speed = 100 - 4 = 96 km/h. Length of train L = relative speed × time = 96 × (15/3600) km = 96 × 15/3600 km = 0.4 km = 400 m. When P moves in opposite direction, relative speed = 100 + 4 = 104 km/h. Time to cross = L / relative speed = 0.4 / 104 hours = 0.4 × 3600 / 104 seconds = 1440/104 ≈ 13.846 seconds ≈ 13.85 seconds.