Pipes A and B can fill a tank in 6 hours and 9 hours respectively while another outlet pipe can empty it in 12 hours. If all the pipes are opened together in the empty tank, in how much time will it be full.

• A.  \[5\frac{1}{7}\]hours    
• B.  \[6\frac{1}{7}\] hours       
• C.  \[8\frac{1}{7}\]hours                
• D.  \[10\frac{1}{7}\]hours