You are a spy in a hostile country, trying to escape the local authorities. As a disguise, you join the local Knavy, which is like a navy, but is made up entirely of knaves (people who alternate telling the truth and lying). You get word that a ship, the HMS Levik is docked in the harbor to help you escape. The problem is, there are five ships in port (at docks one through five) and none of them are marked with a name.
  You manage to talk with two old sea captains in the Knavy about which ship is which. They talk some, but you don't know if their first statement is a truth or a falsehood.
  Based on their curious descriptions, can you figure out which ship is the HMS Levik?

CAPT. HUDSON:
1. The Salamander is not moored next to the Revenge.
2. The Antimony is moored at dock one.
3. The Levik is moored at dock two.
4. The Defiance is moored next to the Levik

CAPT. BAFFIN
1. The Defiance is not moored at dock two.
2. The Levik is moored at dock four.
3. The Salamander is moored next to the Revenge.
4. The Antimony is not moored next to the Salamander.

Just for the moment, let's assume that Hudson spoke the truth on statement 1 and 3. If Hudson's 3rd statement (H3) is true, then Baffin's 2nd statement (B2) must be false meaning that B1 and B3 would be true. However, if H1 is true, then B3 must be false. This leads to a contradiction. Therefore, since H1 and H3 cannot both be true H2 and H4 are true.

Since H2 and H4 are true, H1 must be false. If H1 is false, then the opposite (contrapositive?) must be true, that is, B3 must be true, and by association, B1 must also be true.

H2 truthfully says that Antimony is moored in dock 1.
B4 is a lie, therefore the Salamander is moored in dock 2.
B3 is true, so the Revenge is moored in dock 3.
B2 is a lie, so the Levik cannot be at dock 4, therefore the Defiance is in dock 4 and the Levik is moored in dock 5.

Tot je dienst!

Posted by Erik on 2004-06-01 22:46:42