Quantitative Aptitude :: Problems on Trains - Discussion

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Explanation:
Convert the Speed in m/sec
so 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.

Here assume the speed of the train as x m/sec.
so relative will be (x-1.25) and (x - 1.5)

(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 m/s
As options are given in km/hr so convert the speed mps to kmph
So Speed of the train = ( 22.5 x 18/5 ) km/hr = 81 km/hr.