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      Question

      Train 'M' can cross a pole in 10 seconds and a 200 metres

      long bridge in 15 seconds. Train 'N' whose length is 50% of the length of train 'M' runs in the same direction of train 'M'. If train 'N' crosses train 'M' in 15 seconds, then find the speed of train 'N'.
      A 72 m/s Correct Answer Incorrect Answer
      B 76 m/s Correct Answer Incorrect Answer
      C 80 m/s Correct Answer Incorrect Answer
      D 84 m/s Correct Answer Incorrect Answer
      E 88 m/s Correct Answer Incorrect Answer

      Solution

      Let the length and the speed of the train 'M' be 'x' metres and 's' m/s.
      ATQ,
      (x/s) = 10
      So, 'x' = 10s.........(i)
      Also, {(x + 200) ÷ s} = 15
      Or, 10s + 200 = 15s [from equation (i)]
      Or, 200 = (15s - 10s)
      Or, 's' = (200/5) = 40
      Length of train 'M' = (10 × 40) = 400 metres
      So, length of train 'N' = (400 × 0.50) = 200 metres
      Let the speed of train 'N' be 'n' m/s.
      ATQ,
      (400 + 200) ÷ (n - 40) = 15
      Or, 600 = 15 × (n - 40)
      Or, (n - 40) = 40
      So, 'n' = 40 + 40 = 80
      Therefore, speed of train 'N' = 80 m/s

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