Quantitative Aptitude :: Problems on Trains - Discussion

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Explanation:
Convert the persons Speed in m/sec
2 kmph = (2 x 5/18) m/sec = 5/9 m/sec.
4 kmph = (4 x 5/18 m/sec = 10/9 m/sec.

Let the length of the train be x meters and its speed by y m/sec.
Relative speed in respect to both person (y - 5/9) and (y - 10/9)

Formula Used: Time = Distance / speed
so x / (y - 5/9) = 19 ---- (1)
and x / (y - 10/9) = 20 ---- (2)
9x = 19 (9y - 5) ----- (1)
9x = 20 (9y - 10) ------(2)
find the value of x and y
19 (9y - 5) = 20(9y - 10)
171 y - 95 = 180 y -200 => 9y = 105 => y = 105/9 = 35/3

Now find the value fo x by above any equations
9x = 171 *35/3 - 95 = 57 *35 - 95
9x = 1900 => x = 211.1 meter