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Professor Fukanō, the famous [br]eccentric scientist and adventurer,

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has embarked on a new challenge:

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flying around the world nonstop[br]in a plane of his own design.

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Able to travel consistently at the[br]incredible speed of one degree longitude

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around the equator per minute,

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the plane would take six hours[br]to circle the world.

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There's just one problem:

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the plane can only hold 180 kiloliters[br]of fuel,

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only enough for exactly half the journey.

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Let's be honest.

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The professor probably could have[br]designed the plane to hold more fuel,

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but where's the fun in that?

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Instead, he's devised a slightly more[br]elaborate solution:

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building three identical planes[br]for the mission.

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In addition to their speed,

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the professor's equipped them[br]with a few other incredible features.

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Each of the planes can turn on a dime

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and instantly transfer any amount[br]of its fuel to any of the others in midair

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without slowing down,

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provided they're next to each other.

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The professor will pilot the first plane,

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while his two assistants Fugōri[br]and Orokana will pilot each of the others.

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However, only one airport,[br]located on the equator,

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has granted permission for the experiment,

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making it the starting point,

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the finish line,

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and the only spot where [br]the planes can land,

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takeoff,

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or refuel on the ground.

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How should the three planes coordinate

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so the professor can fly continuously[br]for the whole trip and achieve his dream

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without anyone running out [br]of fuel and crashing?

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Pause here if you want [br]to figure it out for yourself.

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Answer in: 3

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Answer in: 2

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Answer in: 1

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According to the professor's calculations,

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they should be able to pull it off[br]by a hair.

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The key is to maximize the support[br]each assistant provides,

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not wasting a single kiloliter of fuel.

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It also helps us to think symmetrically

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so they can make shorter trips[br]in either direction

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while setting the professor up for a long[br]unsupported stretch in the middle.

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Here's his solution.

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All three planes take off at noon[br]flying west,

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each fully loaded with 180 kiloliters.

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After 45 minutes, or one-eighth[br]of the way around,

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each plane has 135 kiloliters left.

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Orokana gives 45 to the professor[br]and 45 to Fugōri,

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fully refueling them both.

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With her remaining 45, [br]Orokana returns to the airport

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and heads to the lounge[br]for a well-deserved break.

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45 minutes later, with one-quarter[br]of the trip complete,

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the professor and Fugōri[br]are both at 135 kiloliters again.

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Fugōri transfers 45 [br]into the professor's tank,

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leaving himself with the 90[br]he needs to return.

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Professor Fukanō stretches[br]and puts on his favorite album.

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He'll be alone for a while.

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In the meantime, Orokana has been[br]anxiously awaiting Fugōri's return,

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her plane fully refueled and ready to go.

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As soon as his plane touches the ground,[br]she takes off, this time flying east.

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At this point, exactly 180 minutes [br]have passed

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and the professor is at the halfway point[br]of his journey

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with 90 kiloliters of fuel left.

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For the next 90 minutes,

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the professor and Orokana's planes [br]fly towards each other,

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meeting at the three-quarter mark.

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Just as the professor's fuel [br]is about the run out,

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he sees Orokana's plane.

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She gives him 45 kiloliters [br]of her remaining 90,

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leaving them with 45 each.

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But that's just half of what they need[br]to make it to the airport.

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Fortunately, this is exactly when Fugōri,[br]having refueled, takes off.

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45 minutes later, just as the other two[br]planes are about to run empty,

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he meets them at the 315 degree point

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and transfers 45 kiloliters of fuel[br]to each, leaving 45 for himself.

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All three planes land at the airport[br]just as their fuel gauges reach zero.

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As the reporters and photographers cheer,

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the professor promises his planes will[br]soon be available for commercial flights,

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just as soon as they figure out how [br]to keep their inflight meals

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from spilling everywhere.