Four suspects—Ali, Bilal, Daoud, and Samer—are questioned. Exactly one of them is guilty, and exactly one of them is telling the truth.
Ali says: 'Bilal is guilty.'
Bilal says: 'Daoud is guilty.'
Daoud says: 'Bilal is lying.'
Samer says: 'I am not guilty.'
Who is guilty?

Answer and explanation

Correct answer: A. Samer

  1. Step 1: Note that Bilal and Daoud make contradictory statements ('Bilal is lying' vs 'Daoud is guilty').
  2. Step 2: Therefore, exactly one of Bilal or Daoud must be telling the truth.
  3. Step 3: Since exactly one person tells the truth overall, that truthful person must be either Bilal or Daoud.
  4. Step 4: Consequently, both Ali and Samer MUST be lying.
  5. Step 5: Analyze Samer's lie: Samer says 'I am not guilty.' Since this is false, Samer is guilty!
  6. Step 6: Verify: If Samer is guilty, then Bilal is not guilty (Ali lies), Daoud is not guilty (Bilal lies), Daoud tells truth ('Bilal is lying' is TRUE), Samer lies. Exactly one truth and one guilty.
Answer
Samer.
Why not the others: 'Ali' being guilty would make Samer's statement truthful alongside Daoud's statement, violating the premise of only one truth-teller; 'Bilal' being guilty makes Ali's statement true; 'Daoud' being guilty makes Bilal's statement true.

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