4
edits
Changes
Here is a solution involving only three planes, including the one around the world (call this one the "main plane"). Two planes (Plane 1 and Plane 2) escort the main plane, all fully fueled. At 1/8 of the way around the world, Plane 1 fills up Plane 2 and the main plane, then returns to the island. At 1/4 of the way, Plane 2 fills up the main plane and returns to the island. At half way around, Plane 1, fully fueled, takes off again, now in the opposite direction as previously. When it meets the main plane, which is now 3/4 of the way around the world, Plane 1 gives the main plane 1/4 of a tank. They both fly toward the island, and a fully fueled Plane 2 leaves the island, flying toward them. Halfway through their return (so the main plane is now 7/8 the way around the world), Plane 2 meets them and gives each 1/4 of a tank. All three planes can now safely return to the island.
}}
[[Category: Optimization puzzles]]
[[Category: Geometry]]