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 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.

• 5 planes are required
  Divide the world as 6 parts.
  To cover 1/6 th of the distance 1/3 rd of fuel is required.
  1 plane, B, can start in same direction of target flight say A.
  It can travel 1/6 th of the distance and give 1/3rd of fuel to A and return home with 1/3rd fuel. Now A can cover 4/6th world with that full tank.
  Let C and D start from home at opposite direction.
  Let C cover 1/6th dist and give 1/3rd fuel to D. Now D travels 1/3 rd more distance and meets A and gives 1/3rd fuel. Both are now 1/6th distance away from home. Let E take fuel and bring one plane home and F take fuel and bring home A. Now excluding A.. 5 supporting planes are required.
• 9 years agoHelpfull: Yes(14) No(3)
• 4 planes needed
  
• 10 years agoHelpfull: Yes(10) No(7)
• 3 planes needed
• 10 years agoHelpfull: Yes(5) No(5)
• The minimum is three.
  
  Start three planes from the island (yours and two others). After going 1/8 around, one plane refuels both yours and the other plane with 1/4 of a tank each, leaving it with 1/4 of a tank which is precisely enough to take it home.
  
  Two planes continue with a full tank again.
  
  Reaching 1/4 around the other plane transfers 1/4 tank of fuel to yours, leaving it with 1/2 tank, to take it precisely home again.
  
  Your plane is now full at 1/4 and can go to 3/4 with that fuel.
  
  The first returning plane refuels and start off the other way meeting your plane at its 3/4 position, where your plane is empty and the meeting one is half empty. Sharing their fuel they have both 1/4 tank left which can take them to the 7/8 position.
  
  The remaining plane has arrived there in the meanwhile with 3/4 tank giving 1/4 to the other now empty planes.
  
  All three planes have now 1/4 and can return home.
  
  What a trip!
  
• 8 years agoHelpfull: Yes(3) No(0)
• 3 Planes is enough. Assume the 1st plane(A) can go the half of world.if two planes (B& C)supported at its half of distance, it can come to same land.
• 10 years agoHelpfull: Yes(2) No(6)
• 2 planes are enough.
  1. Will give fule at 25 percent of ditance and 1 at 75 percent of distance as earth is sphere.
• 9 years agoHelpfull: Yes(2) No(1)