Pipes P and Q can fill a tank in 18 and 27 minutes, respectively, whereas Pipe R can empty the full tank in 54 minutes.  P and Q were opened together for 6 minutes and then   closed and R was opened. The tank was emptied by R alone in :

A tap can fill the tank in 8 hours and another can empty it in 16 hours. If both the taps are opened simultaneously, the time (in hours) to fill the tank is__.