Speed of Train B = Sb = 1.25 x Sa = 1.25 x 40 = 50 Km/hr
In half an hour, Train A would have mover = = 20 Km away from train C .
Therefore, train C will have to cover 20 Km in relative speed to cross train A. So, time taken = (Sc - 40 Km/hr is the relative speed of train C w.r.t A)
Similarly, train C will have to cover = 25 Km in relative speed to cross train B. So, time taken = .
Given the difference between these two times is 90 minutes. Therefore, .
Now, instead of solving this equation, one can quickly put in the options in this equation to find Sc = 60 Km/hr.
Note - You should be able to rule out A and B beforehand because speed of train Chas to be greater than Train A and B to overtake them.
Correct Answer: 60 Km/hr