Concept: The question tests on the basics of Rates(Speed and Time)
Let the length of the faster train = x m
Here we use Relative speed as there are 2 objects in motion simultaneously.
Since the objects are traveling in the same direction, the relative speed is the difference of speeds = (70 - 50) * 5/18 = 100/18 = 50/9 m /s
Time = D / RS
36 = x / (50/9)
x = 36 * 50/9 = 200 m (option d)