A train covered a certain distance at a uniform speed. If the train had been 6 km/hr faster, then it would have taken 4 hours less than the scheduled time. And, if the train were slower by 6 km/hr, then the train would have taken 6 hours more than the scheduled time. The length of the journey is

Let the actual distance travelled by train be 'd'. Similarly, actual speed and time be 's' and 't'

Given, If the train had been 6 km/hr faster, then it would have taken 4 hours less than the scheduled time,

st = (6 + s)(t - 4) (or) 6t - 4s -24 = 0................(1)

If the train were slower by 6 km/hr, then the train would have taken 6 hours more than the scheduled time,

st = (s - 6)(t + 6) (or) 6t - 6s + 36 = 0...............(2)

Subtract equation (1) and (2)

2s = 60 (or) s = 30

Substitute the value of 's' in equation (1)

6t - 4(30) = 24 (or) 6t = 144 (or) t = 24

The length of the journey (or) total distance = 24 x 30 = 720

Hence, option C is the correct answer.