Correct Answer - Option 3 : 15 sec
Given:
Speed = 72 km/h
Time to cross a man = 9 seconds
Length of platform = 120 m
Formula used:
Speed = distance/time
Calculation:
Speed = 72 × 5/18 = 20 m/s
The distance traveled to cross a man is equal to the length of the train.
∴ D = 20 × 9 = 180 m
Train's length = 180 m
The distance traveled to cross a platform is equal to the length of the train + the length of platform.
∴ D = 180 + 120 = 300 m
T = 300/20 = 15 sec
∴ The answer is 15 sec.