Each plane has only one fuel tank, and a box of oil can be used for an airplane to fly around the Earth for half a lap. There is no fuel dispenser in the air, but the planes can refuel each other.

Each plane has only one fuel tank, and a box of oil can be used for an airplane to fly around the Earth for half a lap. There is no fuel dispenser in the air, but the planes can refuel each other.
Q: How many planes are required to take at least one plane round the earth and return to the airport when it starts to fly?
(All planes take off from the same airport and are not allowed to land in the middle. All planes must return to the airport safely)

Six racks are sufficient.
Three aircraft A, B, and C take off at the same time and travel in the same direction.
Flying to a 1/8 flight (around the Earth for a week), it consumes exactly 1/4 of the fuel, of which 1 aircraft a splits the 2x1/4 fuel tank into the B and C fuel tanks, then, with 1/4 fuel, the system can return safely. At this time, the tank B and C of the plane are full.
The remaining two planes continue to fly 1/8 flights, that is, 1/4 flights. At this time, aircraft B and C also have 3/4 fuel tanks. Aircraft B adds 1/4 fuel tanks to aircraft C, and aircraft B returns the remaining 1/2 fuel tanks. Aircraft C is full of fuel tanks and continues global flights. It can fly to 3/4 flights.
The plane C flew to its flight range of 1/2. Three planes D, E, and F were taken from the same airport at the same time. The plane A, B, and C flew in the opposite direction.
When the aircraft D, E, and F reversely fly for 1/8 flights, the aircraft D adds 2 × 1/4 fuel tanks to the aircraft E and F for return, and the aircraft E and F are full fuel tanks, another 1/8 flight forward, that is, the reverse 1/4 flight, and the forward 3/4 flight, are just at the same time confluence with the global aircraft C, and the aircraft C has run out of fuel, both aircraft E and F have 3/4 fuel tanks. The two planes respectively add 1/4 fuel tanks to aircraft C and return to the airport together with aircraft C, at this time, the aircraft C, E, and F are both 1/2 fuel tanks and can fly the remaining 1/4 flights.
If the planes D, E, and F do not take off at the same time, they can save some fuel, but the departure racks are the same.

Each plane has only one fuel tank, and a box of oil can be used for an airplane to fly around the Earth for half a lap. There is no fuel dispenser in the air, but the planes can refuel each other.