From pole to bridge – finding total crossing time A 150 m train passes a telegraph post in 12 seconds. In how many seconds will it pass a 250 m bridge?
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A32 sec
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B36 sec
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C25 sec
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D24 sec
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ENone of these
Answer
Correct Answer: 32 sec
Explanation
Introduction / Context:Speed from a pole crossing yields the train’s speed because only its own length is traversed. That speed then applies to longer structures like bridges, where the distance includes both lengths combined.
Given Data / Assumptions:
- Train length = 150 m.
- Time to pass pole = 12 s.
- Bridge length = 250 m.
Concept / Approach:First determine speed from pole data. Then compute time for total distance (train + bridge).
Step-by-Step Solution:
v = distance / time = 150 / 12 = 12.5 m/sTotal distance D = 150 + 250 = 400 mt = D / v = 400 / 12.5 = 32 sVerification / Alternative check:Multiply back: 32 s * 12.5 m/s = 400 m, matching train + bridge lengths.
Why Other Options Are Wrong:24 s, 25 s, 36 s: Each yields inconsistent distances (300 m, 312.5 m, 450 m), not equal to 400 m.
Common Pitfalls:Using only bridge length in D; the correct total is train + bridge. Also, beware of rounding speed—here it is exact (12.5 m/s).
Final Answer:32 sec