Tried to map the logical sequence of events.
1. Bus 1 left, travelling at 50km/hr. Bus 2 and 3 are still stationary
2. Bus 2 left, 6 hours after Bus 1, at 80 km/h: At this point, Bus 1 would have already 300 kms (50 km/hr * 6 hrs = 300 kms).
3. Bus 3 has also left at some point, travelling at 100 km/hr. After travelling for some time, Bus 3 along with Bus 2, catchup to Bus 1 at the same time.
Now since Bus 2 and 3 catch up at the same time, we need to find this time and that should help us get when Bus 3 left exactly.
To find this time, we can use the Gap Distance Concept.
Relative Speed = (Gap Distance b/w the at the point the bodies) / (Total time for which both bodies move simultaneously, denoted going forward by T)
We know that Bus 1 had already travelled 300 kms by the time Bus 2 started moving (i.e. they both started moving simultaneously)
Since they are moving in the same direction, Relative Speed will be the difference of their speeds i.e. 80 - 50 = 30 km/hr
Thus, the above formulat becomes
30 = 300/T
Thus, T = 10 hours.
So the Buses crossed each other 10 hours after Bus 2 started moving (i.e. 16 hours after Bus 1 started moving)
Now in these 16 hours, Bus 1 would have travelled 16*50 = 800 kms
Hence, Bus 2 and Bus 3 catch up with Bus 1 when Bus 1 has travelled 800 kms.
For Bus 3 to travel 800 kms it would take 800/100 = 8 hours (Time = Distance/Speed)
Thus, Bus 1 travelled for 16 hours, and Bus 3 travelled for 8 hours.
This means Bus 3 left 8 hours after Bus 1.
Answer (C)