FLY AROUND
THE WORLD
There are a number of planes on an island. The fuel tank of each plane, when filled, contains enough fuel to take it halfway around the world. It is possible to transfer fuel from one plane to another during flight, but there is no other source of fuel available. The pilots are told that one plane must fly around the world and that every plane must return to the island. What is the absolute minimum number of planes needed so that one of them can fly around the world? Explain your answer.