66.
Pipe A takes 4/5 of the time required by pipe B to fill an empty tank individually.
When an outlet pipe C is also opened simultaneously with pipes A and B, it takes 4/5 more time to fill the empty tank than it takes when only pipe A and pipe B are opened together. If it takes 40 hours to fill the tank when all the three pipes are opened simultaneously, in what time (in hours) will pipe C empty the full tank, operating alone?