Two persons are walking beside a railway track at respective speeds of 2 and 4 km per hour in the same direction. A train came from behind them and crossed them in 90 and 100 seconds, respectively. The time, in seconds, taken by the train to cross an electric post is nearest to
Explanation:
Let the speed and length of train be ‘s’ m/s and ‘t’ m respectively.
Speeds of two persons is 2 kmph and 4 kmph i.e., 5/9 m/s and 10/9 m/s
Train passes the first person in 90 seconds.
∴ ts-59 = 90
⇒ t = 90s – 50 …(1)
Train passes the second person in 100 seconds.
∴ ts-109 = 100
⇒ t = 100s – 1000/9 …(2)
(1) = (2)
⇒ 90s – 50 = 100s – 1000/9
⇒ s = 55/9 m/s and t = 500 m
∴ Time taken to cross an electric pole = 50055/9 = 450055 = 90011 = 81.81 ≈ 82 seconds.
Hence, option (b).
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