A large gasoline tank has
three inlet pipes and one outlet pipe. One inlet pipe, colored red
takes 2 hours to fill the tank. Another inlet pipe, colored
blue, takes 3 hours to fill the tank. The third inlet pipe, colored
green, takes 4 hours to fill the tank. The outlet pipe empties the
whole tank in 12 hours.
If the tank is initially empty, how long would it
take to fill the tank if all the pipes are fully in operation?