a train met with accident 120 km from station A. it completed the remaining journey at 5/6 of its previous speed and reached 2 hours late at station B. had the accident take place 300 km further , it would have been only 1 hour late, what is the speed of the train

This is one of the famous models in Time and Distance.
Given that train met with an accident at 120 km from station A
We know that Speed x Time = Distance
So based on Product constancy, If speed becomes 5/6 then time taken will increase by 6/5 of its original time.
Given, extra time = 2 hours
(6t/5 - t) = ⅕t = 2 hours
Therefore, t = 10 hrs
Usual time which is required to complete the journey = 5*2 = 10 hours.
That means It covers 300 km distance in 5 hours.
So, speed = 300/5 = 60 kmph.