It's possible to drive completely around the desert planet Flargg at its equator, a distance of 10,000 kilometers, since the only water on the planet is in a lake that is just to the north of the equator. The problem is that Flarggian water-powered vehicles can only transport two tanks of water at a time. One tank is sufficient to travel 2000 kilometers. In order to make a trip around Flargg, Flarggians make extra trips to drop off spare water tanks, which can be left safely anywhere along their route.
What is the most efficient way to make a trip around Flargg, beginning and ending at the lake? (The most efficient way is the way that requires the shortest total distance travelled, including all extra trips to drop off spare water tanks.)