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      Question

      A train can cross a bridge and a pole in 7.2 seconds and 4

      seconds respectively. The length of the bridge is 80 metres. Find the time taken by train to cross a car which is running in the same direction with a speed of 20 m/sec.
      A 22 seconds Correct Answer Incorrect Answer
      B 20 seconds Correct Answer Incorrect Answer
      C 25 seconds Correct Answer Incorrect Answer
      D 18 seconds Correct Answer Incorrect Answer

      Solution

      Let the length and the speed of the train be 'x' metres and 'y' m/sec respectively.
      When it crosses the bridge,
      Or, {(x + 80) / y} = 7.2.................(I)
      When it crosses the pole,
      Or, (x / y) = 4
      So, x = 4y............(II)
      Or, 4y + 80 = 7.2y
      Or, 80 = 3.2y
      So, y = (80 / 3.2) = 25 m/sec
      And x = 25 × 4 = 100 metres
      So, relative speed of the train w.r.t. the car = 25 - 20 = 5 m/sec
      So, required time = (100 / 5) = 20 seconds

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