Explanation:

Let the length of the journey be d km and the speed of train be SS km/hr.

Then,

dS+6=t−4dS+6=t−4-----(i)

dS−6=t+6dS−6=t+6------(ii)

Subtracting the 1 equation from another we get:

dS−6−dS+6=10dS−6−dS+6=10------(iii)

Now t=dst=ds

Substitute in equation (i) and solve for dd and SS

We get S=30S=30 and  dd = 720 km

Hence, option 3 is the correct answer.