Professor Fiendish plans to fly all the way around the equator, a journey of 40,000km.
He starts and finishes at the same airport.
He has three aeroplanes, but unfortunately each plane only holds enough fuel to go 20,000km (which is halfway)!
Luckily he has two enthusiastic assistants to fly the other planes and help him...
The planes can transfer fuel between them at any time. The planes can return home to refuel when necessary.
All three planes must finish safely at the airport!
How can the professor fly all the way around the world?