Question
Two trains ‘D’ and ‘E’ of lengths 660 metres and 440 metres respectively are travelling towards each other. Ratio of speed of ‘D’ to ‘E’ is 3:2. If the trains take 55 seconds to cross each other from the time they meet, then find the average of speeds (in km/h) of trains ‘D’ and ‘E’.
Solution
Let the speeds be 3y and 2y m/s respectively.
Relative speed = 3y + 2y = 5y m/s
ATQ,
5y × 55 = 660 + 440
⇒ 275y = 1100
⇒ y = 4
Therefore, required average = (3y + 2y) ÷ 2 = 2.5y
= 2.5 × 4 = 10 m/s = 10 × (18/5) = 36 km/h
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