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

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 :
$$\eqalign{ & = \frac{{125{\text{ m/s}}}}{{18{\text{ kms}}}} \cr & = \frac{{125{\text{ }} \times {\text{18}}}}{{18 \times 5{\text{ }}}}{\text{m/s}} \cr & \left\{ {\because 1{\text{ km/hr = }}\frac{5}{{18}}{\text{ m/s}}} \right\} \cr & = 25\sec \cr} $$