How much time does a train 50 m long, moving at 68 km/hr takes to pass another train 75 m long moving at 50 km/hr in the same direction ?

Answer: Option D

Total distance covered by the length of both trains :
= 50 m + 75 m
And, their relative speed in same direction :
= 68 - 50 = 18 km/hr
$$\because$$ (Speed subtracted in same direction)
Then, the time to cross each other will be :
$$\begin{array}{rl}& =\frac{125\text{ m/s}}{18\text{ kms}}\\ & =\frac{125\times \text{18}}{18\times 5}\text{m/s}\\ & \left\{\because 1\text{ km/hr = }\frac{5}{18}\text{ m/s}\right\}\\ & =25\sec \end{array}$$