A. 3 9/17 hours
Feedback Pipes A and B can fill the tank in 5 and 6 hours respectively. Therefore,
part filled by pipe A in 1 hour =1/5
part filled by pipe B in 1 hour = 1/6
Pipe C can empty the tank in 12 hours. Therefore,
part emptied by pipe C in 1 hour =1/12
Net part filled by Pipes A,B,C together in 1 hour=1/5+1/6-1/12=17/60
i.e., the tank can be filled in 60/17=3 9/17 hours.