Correct Answer - Option 2 : 4 ∶ 3
Given:
Time taken by A to cross the man = 20 sec
Time taken by B to cross the man = 1 min 20 sec = 80 sec
Length of train A = 1/3 × length of train B
Formula used:
Time = Distance/Speed
Concept used:
If any train crosses a man/pole or any other vertical object, distance travelled by train is equal to length of the train
Calculations
Let speed of train A be V1 m/sec
Distance travelled by A to cross the pole = 20V1
Length of train A = 20V1
Length of train B = 3 × length of A = 3 × 20V1 = 60V1
Speed of train B = 60V1/80
⇒ 3V1/4
Ratio of speeds = V1/(3V1/4) = 4/3
∴ Required ratio of speeds is 4 ∶ 3