(A) 66 km/hr
(B) 72 km/hr
(C) 78 km/hr
(D) 81 km/hr
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(A) 66 km/hr
(B) 72 km/hr
(C) 78 km/hr
(D) 81 km/hr
Answer: Option (D) 81 km/hr
Explanation:
4.5 km/hr = 4.5 x (5/18) m/sec = 5/4 m/sec = 1.25 m/sec, and
5.4 km/hr = 5.4 x (5/18) m/sec = 3/2 m/sec = 1.5 m/sec.
Let the speed of the train be x m/sec.
Then, (x – 1.25) x 8.4 = (x – 1.5) x 8.5
=> 8.4x – 10.5 = 8.5x – 12.75
=> 0.1x = 2.25
=> x = 22.5
Therefore Speed of the train = 22.5 x 18/5 km/hr = 81 km/hr.