Correct Answer - Option 2 : 400 m.
Given:
Length of bridge 1 = 400 m
Time taken by train to cross bridge 1 = 80 seconds
Length of bridge 2 = 200 m
Time taken by train to cross bridge 2 = 60 seconds
Formula Used:
Distance = Speed × Time
Calculation:
Let assume that the speed of the train is y m/sec.
And, length of the train = x m
Case 1:
Total distance = 400 m + x m
Time = 80 seconds
Speed = Total distance/ Time
⇒ y = (400 + x)/80 ---- (1)
Case 2:
Total distance = 200 m + x m
Time = 60 seconds
Speed = Total distance/ Time
⇒ y = (200 + x)/60 ---- (2)
From equation (1) and equation (2)
⇒ (400 + x)/80 = (200 + x)/60
By cross multiplication
⇒ x = 400 m
∴ The length of the train is 400 m.
