There is one highland where planes get fuel. A plan can cover the half of the world with full tank. A plane can transfer fuel from one plane to the other in the air.
How many number of planes are required to make one plane cover the entire world once. Starting point of the travel is the Highland and coming back to the same Highland.
All the other planes giving fuels to this Plane should also come back to the same highland and they don't have to cover the entire world.