Question:

A dragon and a knight are trapped on a desert island. This island has seven poisoned wells, numbered 1 to 7. If they drink from a well, they can save themselves only by drinking from a higher number well within a few minutes’ time. Well 7 is located at the top of a high mountain, so only the dragon can reach it. In other words, if the dragon drinks from well 4, it will die unless it has a cup of poison from wells 5 to 7. The same is true of the knight. One day they decide that the island isn’t big enough for the two of them, and they have a duel. Both the knight and dragon are rational creatures. Each of them brings a cup of water to the duel, they exchange cups, and both drink the contents of the cup brought by his adversary. After the duel, the knight lives and the dragon dies. What strategy did the knight use to defeat the dragon?

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C

A

P

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A

I

N

I

N

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E

R

V

I

E

W

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Solution: Let’s review the facts. Both the dragon and the knight know that the knight cannot reach well 7. The dragon therefore probably fills its cup from well 7 knowing that the knight could not get an antidote to the water from that well. The knight figures this out and so prepares himself with a cup of water from well 1. First he drinks water from well 1 (weakest poison), and then he quickly drinks water from the dragon’s glass, which probably contains water from a higher number well. Then he quickly drinks from the well 1 cup again, and then he quickly drinks water from well 2.

That way, if the dragon offered him water from wells 2 to 7 the knight simply poisoned and cured himself twice. If the dragon gave him water from well 1, then what actually happened was that he drank water from well 1 and then water from well 2—which would save him. For his strategy, the knight gives the dragon a cup filled from water surrounding the island. The dragon, thinking it got a cup of poison from well 1 (or up to well 6), drank it and then immediately drank from well 7, as an antidote to a presumed poisoning. But since the dragon, in fact, drank seawater and not poisoned water, drinking from well 7 poisoned it fatally.

Answer: Give the dragon sea water.