Question: 150 meter long train crosses a platform
of length 250 meters in 30 seconds. Find the time for train to cross a bridge
of 130 meters. [150 मी लम्बी ट्रेन, 250 मी लम्बे प्लेटफार्म को
30 सेकंड में पार कर लेती है. 130 मी लम्बे ब्रिज को पार करने में उस ट्रेन को कितना
समय लगेगा]. [Ans. (c) 21 second]
(a)12 sec (b)
42 sec. (c) 21 sec (d) 22 sec (e) None
of these
Detailed Explanation:
Concept: When a train
crosses an electric pole or a person, it describes a distance equal to its
length. But, when a train crosses a platform or bridge; it describes a distance
equal to the sum of the length of train and platform/bridge.
Relative speed is sum of
the speeds of both the moving objects, when they move in opposite directions;
but it is equal to the difference of the speeds of both the moving objects,
when they move in the same directions.
First Method: Here, 150
meter long train crosses a platform of length 250 meter, so the distance is
equal to the sum of the lengths of train and platform, that is 150+250 = 400
meter. And, time is 30 seconds. Again, when the train crosses a bridge of 130
meter, then the distance = 150+130 = 280 meter. The speed is constant, as
moving body in both the cases is the same train.
So, 400 m ≡ 30 sec
280 m ≡? = 21 sec Ans.
Second Method: As the
moving body in both the cases is the same train, so speed is constant. In this
case the ratio of timings is equal to the ratio of distances.
That is, T1:T2 = S1:S2
= 400:280 = 10:7
10 ≡ 30 sec, 7 ≡? = 21
sec. Ans.
Solution: First Method
400 m ≡ 30 sec
280 m ≡? = 21 sec Ans.
 |
Second Method
T1:T2 = S1:S2 = 400:280 = 10:7
10 ≡ 30 sec, 7 ≡? = 21 sec. Ans. 
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