Two pipes A and B can fill a tank in 20 and 30 hours respectively. Both the pipes are opened to fill the tank but when the tank is one-third full, a leak develops in the tank through which one-third water supplied by both the pipes goes out. The total time taken to fill the tank is `12\ hou r s` b. `14\ hou r s` c. `16\ hou r s` d. `18\ hou r s`