Pipe A can fill a tank in 20 hours and pipe B can empty the full tank in 40 hours. Both pipes were initially opened together for 30 hours,…

Pipe A can fill a tank in 20 hours and pipe B can empty the full tank in 40 hours. Both pipes were initially opened together for 30 hours, and then pipe B was closed and pipe A continued to fill the tank. In how much total time (in hours) was the tank filled completely?