Question
Two trains ‘L’ and ‘M’ of lengths 720 metres and 480 metres respectively are travelling towards each other. Ratio of speed of ‘L’ to ‘M’ is 5:3. If the trains take 60 seconds to cross each other from the time they meet, then find the average of speeds (in km/h) of trains ‘L’ and ‘M’.
Solution
Let the speeds be 5y and 3y m/s respectively.
Relative speed = 5y + 3y = 8y m/s
ATQ,
8y × 60 = 720 + 480
⇒ 480y = 1200
⇒ y = 2.5
Therefore, required average = (5y + 3y) ÷ 2 = 4y
= 4 × 2.5 = 10 m/s = 10 × (18/5) = 36 km/h
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