<PRE><B>Question 384750</B>
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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 
 {{{1/n = 2/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</PRE>