A train running at a speed of 20 m/s takes 22 seconds to pass a bridge and it takes 10 seconds to pass a man who is running at a speed of 5 m/s in the same direction. Find the length of the bridge.
- AOption (a)
150 m
- BOption (b)
200 m
- COption (c)
290 m
- DOption (d)
300 m
- EOption (e)
350 m
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Correct answer
290 m
Why this answer is correct
Shortcut Trick
Relative Speed with man = 20 − 5 = 15 m/s.
Length of Train (LT) = Relative Speed × Time = 15 × 10 = 150 m.
Total distance to cross bridge = LT + LB = Train Speed × Time = 20 × 22 = 440 m.
Length of Bridge (LB) = 440 − 150 = 290 m.
∴ The correct answer is 290 m.
Alternate Method
Given:
Speed of train (VT) = 20 m/s
Speed of man (VM) = 5 m/s (Same direction)
Time to pass man (t1) = 10 s
Time to pass bridge (t2) = 22 s
Formula Used:
Distance = Speed × Time
Relative Speed (Same direction) = V1 − V2
Calculations:
⇒ Relative speed of train w.r.t man = 20 − 5 = 15 m/s
⇒ Length of train (LT) = Relative Speed × t1 = 15 × 10 = 150 m
⇒ Total distance to cross bridge = LT + Length of Bridge (LB)
⇒ LT + LB = VT × t2
⇒ 150 + LB = 20 × 22
⇒ 150 + LB = 440
⇒ LB = 440 − 150 = 290 m
∴ The correct answer is 290 m.
Additional Information
Relative Speed
When two objects move in the same direction, relative speed = S1 − S2. In opposite directions, relative speed = S1 + S2.
Crossing Stationary Objects
To cross a point object (pole, man), distance = LTrain. To cross an object with length (bridge, platform), distance = LTrain + LObject.
Unit Conversion
To convert km/hr to m/s, multiply by 5/18. To convert m/s to km/hr, multiply by 18/5.