A train 125 m long passes a man running at 5 km hr in the same direction in which the train is going...........

Explanation:

Speed of the train relative to man = (125/10) m/sec = (25/2) m/sec.
[(25/2) x (18/5)] km/hr = 45 km/hr.
Let the speed of the train be 'S' km/hr.
Then, relative speed = (S - 5) km/hr.
S - 5 = 45 => S = 50 km/hr.

Hence, option 4 is the correct answer.