Question
Two trains ‘P’ and ‘Q’ of lengths 600 metres and 400 metres respectively are travelling towards each other. Ratio of speed of ‘P’ to ‘Q’ is 4:1. If the trains take 50 seconds to cross each other from the time they meet, then find the average of speeds (in km/h) of trains ‘P’ and ‘Q’.
Solution
Let the speed of ‘P’ and ‘Q’ be ‘4y’ and ‘y’ m/s respectively.
Relative speed = 4y + y = ‘5y’ m/s
ATQ,
5y × 50 = 600 + 400
⇒ 250y = 1000
⇒ y = 4
Therefore, required average = (4y + y) ÷ 2 = 2.5y
= 2.5 × 4 = 10 m/s = 10 × (18/5) = 36 km/h
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