Pipe A can fill an industrial tank in 8 hours, and pipe B can fill the same tank in 4 hours. Let x represent the number of hours needed to fill the tank when both pipes are in operation. in one hour, pipe A fills;
1/8 of the tank, pipe B fills 1/4 of the tank, and with both pipes working, 1 of the tank
is filled. Find the time needed to fill the tank wth both pipes working together.