Shortcut Trick
Total distance to cross = Sum of train lengths = 410 + 440 = 850 m.
Relative speed (same direction) = 70 − 60 = 10 km/h.
Convert relative speed to m/s = 10 × (5/18) = 50/18 m/s.
Time in seconds = Distance ÷ Speed = 850 ÷ (50/18) = 850 × (18/50) = 306 seconds.
Time in minutes = 306 ÷ 60 = 5.1 minutes.
∴ The correct answer is 5.1 minutes.
Alternate Method
Train 1 (410 m)
60 km/h
Train 2 (440 m)
70 km/h
Relative Speed = 10 km/h → Total Distance = 850 m
Given:
Length of slower train (L1) = 410 m
Length of faster train (L2) = 440 m
Speed of slower train (S1) = 60 km/h
Speed of faster train (S2) = 70 km/h
Formula Used:
Relative Speed (when travelling in same direction) = S2 − S1
Time Taken = Total Distance Covered ÷ Relative Speed
Calculations:
⇒ Total distance to cross each other = L1 + L2 = 410 + 440 = 850 m
⇒ Relative Speed = 70 − 60 = 10 km/h
⇒ Converting Relative Speed to m/s = 10 × (5/18) = 50/18 = 25/9 m/s
⇒ Time taken (in seconds) = 850 ÷ (25/9)
⇒ Time taken = (850 × 9) ÷ 25
⇒ Time taken = 34 × 9 = 306 seconds
⇒ Converting seconds into minutes = 306 ÷ 60 = 5.1 minutes
∴ The correct answer is 5.1 minutes.
Additional Information
Relative Speed in Opposite Directions
When two objects move towards each other, their relative speed is calculated as the sum of their individual speeds (S1 + S2).
Crossing a Stationary Object (Without Length)
When a train crosses a pole, signal, or a standing man, the distance covered is equal to the length of the train alone.
Crossing a Stationary Object (With Length)
When a train crosses a platform or a bridge, the total distance covered is the sum of the train's length and the platform's length.
Unit Conversions
To convert km/h into m/s, multiply the speed by 5/18. Conversely, to convert m/s into km/h, multiply by 18/5.