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      Question

      Tap β€˜A’ and tap β€˜B’ can fill a tank in 42 hours

      and 30 hours, respectively. If both taps are opened together, then find the time after which tap β€˜A’ must be closed so the whole tank can be filled in 20 hours.
      A 16 hours Correct Answer Incorrect Answer
      B 18 hours Correct Answer Incorrect Answer
      C 14 hours Correct Answer Incorrect Answer
      D 32 hours Correct Answer Incorrect Answer

      Solution

      Let total capacity of the tank is 210 litres (LCM of 42 and 30) Quantity of water filled by tap β€˜A’ in one hour = 210/42 = 5 litres Quantity of water filled by tap β€˜B’ in one hour = 210/30 = 7 litres Let tap β€˜A’ is closed after β€˜x’ hours So, 12 Γ— x + 7 Γ— (20 – x) = 210 Or, 5x = 70 Or, x = 14 hours

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