The train is 125 m or 0.125 km long. It is traveling at x km/hr. But relative to the man it is only going (x-5) km/hr. In 10 seconds (1/360 hr.) it travels (x-5)/360 km relative to the man. So at its speed relative to the man, it travels through 1 train length (0.125 km) in 10 seconds or 1/360 hr.Solve that and get 50 km/hr. Edwin