A train 125 m long passes a man, running at 5 kmph in the same direction in which the train is going, in 10 seconds. The speed of the train is:

Answer: Option B

Speed of the train relative to man
$$\begin{array}{rl}& =\frac{125}{10}\text{m/sec}\\ & =\frac{25}{2}\text{m/sec}\\ & =\left(\frac{25}{2}\times \frac{18}{5}\right)\text{m/sec}\\ & =45\text{km/hr}\end{array}$$
Let the speed of the train be x kmph.
Then, relative speed = (x - 5) kmph
∴ x - 5 = 45 or
x = 50 km/hr