Quantitative Aptitude :: Problems on Trains - Discussion

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Explanation:
Lets assume speed of freight train = X Km/hr
speed of passenger train = Y Km/hr.
and sum of their length = D Km.
When trains are running in same direction:
so Time = D/(X-Y) ---------- (1)
When trains are running in opposite direction:
So, Time = D/(X+Y) --------- (2)
By eq. (1) & (2)
[D/(X-Y)] = 2 * [D/(X+Y)]
=> 2X-2Y = X+Y
=> X = 3Y
=> X:Y = 3:1