A tank is filled in 9 hrs. 

So the fill rate is 1 tank per 9 hrs or

 or ,

and the drain rate is -1 tank per 11 hrs or

 or .

Since the drain is only half open, it's rate is halved

 or 

Their combined rate is  or

since 

their combined fill rate is 

Suppose it takes x hours.  Then

  



 

Edwin