Hi,         

Let n and  n^2  represent time to fill and empty the tank respectively

Question States*** 

  1) BOTH INLET and OUTLET are open with the tank to FILL! AND..

2) Time/hr FILLING the empty tank = Time/hr emptying full tank

 both inlet and outlet valves open

         1/n - 1/n^2              =   1/n^2 

   Cross Multiply

  2n = n^2  

   2hr = n  

The outlet of the full tank is opened at 8 AM, n^2 = 4hrs, the time to empty

Full Tank would be empty at 12 Noon 

and the other Full(with both inlet and outlet open)



CHECKING our Answer***

 1/2 - 1/4 = 1/4 

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