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      Question

      Train 'C' can cross a pole in 6 seconds and a 120 metres

      long bridge in 10 seconds. Train 'D' whose length is 25% of the length of train 'C' runs in the same direction of train 'C'. If train 'D' crosses train 'C' in 15 seconds, then find the speed of train 'D'.
      A 40 m/s Correct Answer Incorrect Answer
      B 42 m/s Correct Answer Incorrect Answer
      C 45 m/s Correct Answer Incorrect Answer
      D 48 m/s Correct Answer Incorrect Answer
      E 50 m/s Correct Answer Incorrect Answer

      Solution

      Let the length and the speed of the train 'C' be 'x' metres and 's' m/s.
      ATQ,
      (x/s) = 6
      So, 'x' = 6s.........(i)
      Also, {(x + 120) ÷ s} = 10
      Or, 6s + 120 = 10s [from equation (i)]
      Or, 120 = (10s - 6s)
      Or, 's' = (120/4) = 30
      Length of train 'C' = (6 × 30) = 180 metres
      So, length of train 'D' = (180 × 0.25) = 45 metres
      Let the speed of train 'D' be 'd' m/s.
      ATQ,
      (180 + 45) ÷ (d - 30) = 15
      Or, 225 = 15 × (d - 30)
      Or, (d - 30) = 15
      So, 'd' = 15 + 30 = 45
      Therefore, speed of train 'D' = 45 m/s

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