How much time will a train of length 200 m, moving at a speed of 72 km/h take to cross another train of length 300 m completely, moving at 36 km/h in the same direction?
- AOption (a)
55 sec
- BOption (b)
50 sec
- COption (c)
100 sec
- DOption (d)
25 sec
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Correct answer
50 sec
Why this answer is correct
Shortcut Trick
Relative Speed in same direction = Difference of speeds = 72 − 36 = 36 km/h.
Convert speed to m/s: 36 × (5/18) = 10 m/s.
Total Distance to cover = Sum of lengths = 200 + 300 = 500 m.
Time = Distance ÷ Relative Speed = 500 ÷ 10 = 50 seconds.
∴ The correct answer is 50 sec.
Alternate Method
Given:
Length of first train (L1) = 200 m
Length of second train (L2) = 300 m
Speed of first train (S1) = 72 km/h
Speed of second train (S2) = 36 km/h
Formula Used:
Time = (L1 + L2) ÷ (S1 − S2)
Conversion: 1 km/h = 5/18 m/s
Calculations:
⇒ Relative Speed = 72 − 36 = 36 km/h
⇒ Relative Speed in m/s = 36 × (5/18) = 10 m/s
⇒ Total distance to be covered = 200 + 300 = 500 m
⇒ Time taken = 500 ÷ 10
⇒ Time taken = 50 seconds
∴ The correct answer is 50 sec.
Additional Information
Relative Speed Concept
When two objects move in the same direction, relative speed is S1 − S2. When they move in opposite directions, relative speed is S1 + S2.
Crossing Distance
To cross each other completely, the distance covered is always the sum of the lengths of both trains, regardless of the direction.
Unit Conversion
To convert km/h to m/s, multiply by 5/18. To convert m/s to km/h, multiply by 18/5.