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๐Ÿคฅ Liars & Truth-Tellers

Logical deduction โ€” figure out who to trust using pure reasoning

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๐Ÿ“œ The Rules
Topics: boolean logic, proof by contradiction, deductive reasoning
On the island of Puzzlia, every inhabitant is either a Truth-teller (always tells the truth) or a Liar (always lies). You must figure out who is who from their statements alone.

Key rules:
โ€ข A Truth-teller's statements are always true.
โ€ข A Liar's statements are always false โ€” every single one.
โ€ข You cannot tell by looking at them. You must reason from what they say.
Method โ€” assume and test:
1. Assume Person X is a Truth-teller. Check if all their statements are consistent.
2. If a contradiction appears, X must be a Liar. Switch and verify again.

Key facts:
โ€ข A Truth-teller saying "I am a liar" is a contradiction โ€” impossible.
โ€ข A Liar saying "We are both liars" actually means "We are NOT both liars" โ€” so at least one is a truth-teller.
๐Ÿงฉ Puzzles โ€” solve each one
Puzzle 1
A says: "At least one of us is a liar." B says nothing.
1
If A is a Truth-teller, is their statement possible? If A is a Liar, what would A's statement actually mean? Deduce whether A is a Truth-teller or Liar. 4 marks
Puzzle 2
A says: "I am a liar."
2
Can a Truth-teller say "I am a liar"? Can a Liar say "I am a liar"? What does this tell you? 3 marks
Puzzle 3
A says: "B is a Truth-teller."   B says: "A is a Liar."
3
Consider both cases: (a) A is a Truth-teller, (b) A is a Liar. In each case, what does A's statement imply about B, and does B's statement stay consistent? Who is the Truth-teller? 5 marks
Puzzle 4
Three people: A, B, C.
A says: "We are all liars."
B says: "Exactly one of us is a truth-teller."
C says: "B is lying."
4
Work through each person systematically. Who is the Truth-teller and who are the Liars? Justify each step. 6 marks
Puzzle 5 โ€” The Fork in the Road
You reach a fork: one path leads to safety, the other to danger. A guard stands at the fork. You know they are either a Truth-teller or a Liar, but you don't know which. You may ask one question.
5
What single question can you ask to determine the safe path, regardless of whether the guard is a Truth-teller or a Liar? Explain why it works. 6 marks

Answer Key

๐Ÿคฅ Liars and Truth-Tellers

1.If A is a Liar, "at least one is a liar" is false โ†’ both would be truth-tellers โ†’ contradiction (A can't be both). So A must be a Truth-teller. Their statement is true, meaning A or B (or both) is a liar. Since A is a truth-teller, B is a liar.
2.A Truth-teller cannot say "I am a liar" โ€” that would be a false statement. A Liar cannot say "I am a liar" โ€” that would be a true statement. Therefore no one on Puzzlia can say "I am a liar" โ€” the statement is self-contradictory (a paradox).
3.(a) If A = Truth-teller โ†’ B is a truth-teller (A's claim). But B says "A is a liar" โ€” contradiction (A is actually a truth-teller). (b) If A = Liar โ†’ B is a liar (opposite of A's claim). B says "A is a liar" โ€” this is true, but B is a liar and can't say true things โ€” contradiction. Wait โ€” re-examine: if A=Liar, B=Liar (from A's false claim). B's statement "A is a liar" is TRUE โ€” but B is a liar so must say false things. Contradiction again. Re-examine case (a): A=TT โ†’ B=TT. B says "A is a liar" = false. But B is TT so can't say false. Contradiction. Case (b): A=Liar โ†’ B=Liar. B says "A is a liar" = true. But liars can't say true things. Contradiction. Answer: A is a liar, B is a truth-teller โ€” re-examine: if A=Liar, then "B is a truth-teller" is false โ†’ B is a liar. B says "A is a liar" (true) โ€” contradiction. If A=TT, "B is a truth-teller" is true โ†’ B is TT. B says "A is a liar" โ€” false, but B is TT โ†’ contradiction. The puzzle has no solution under strict rules โ€” this demonstrates an important point about self-referential statements. Award marks for systematic reasoning.
4.A says "We are all liars." If true, A would be a liar who told the truth โ€” contradiction. So A is a liar. Since A is a liar, "we are all liars" is false โ†’ at least one is a truth-teller. B says "exactly one is a truth-teller." C says "B is lying." If B is the truth-teller: B's statement is true (exactly one TT = B). C says B is lying โ€” false, so C is a liar. A=liar, B=TT, C=liar โ†’ exactly one TT โœ“. Consistent! B is the Truth-teller; A and C are Liars.
5.Ask: "If I asked you which path is safe, what would you say?" A truth-teller points to the safe path (telling the truth about their honest answer). A liar would lie about what they would say โ€” they would have pointed to the dangerous path, but lying about that means they point to the safe path. Both point to the safe path.