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      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’.
      A 27 km/h Correct Answer Incorrect Answer
      B 31 km/h Correct Answer Incorrect Answer
      C 36 km/h Correct Answer Incorrect Answer
      D 45 km/h Correct Answer Incorrect Answer
      E 51 km/h Correct Answer Incorrect Answer

      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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