Speed of a train T is 100 km per hour and that of a person P is 4 km per hour. If P walks in the direction of motion of T, T crosses P in 15 seconds. If P walks in opposite direction to T, in approximately how many seconds will T cross P?

When P walks in same direction: relative speed = 100 – 4 = 96 km/h. Time = 15 s. Length of train L = 96 × (5/18) × 15 = 96 × 5 × 15 / 18 = 7200/18 = 400 m. When P walks in opposite direction: relative speed = 100 + 4 = 104 km/h = 104 × 5/18 m/s = 520/18 ≈ 28.889 m/s. Time = 400 / 28.889 ≈ 13.846 seconds ≈ 13·85 seconds.