correct answer is : c) 6 hours

Answer

Let, the train own speed = x km/hr
According to the question,
In 15 hours, the train reached a place at \frac{2}{5} of its own speed.

We know,
Distance = Speed \times Time

\therefore\ The distance to the destination is
= \left(x\ \times\ \frac{2}{5}\right) \times\ 15 km
= \frac{2x}{5}\ \times\ 15 km
= \frac{30x}{5} km
= 6x km

\therefore Time taken the train to reach its own speed is
= \frac{6x}{x} hours \left[time\ =\ \frac{distince}{speed}\right]
= 6 hours.

Ans: Time taken the train to reach its own speed is 6 hours.