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Consider the following puzzle.

Five persons $\text{A, B, C, D, E}$ are in a compartment in a train. $\text{A, C, E}$ are men, and $\text{B, D}$ are women. The train passes through a tunnel, and when it emerges, it is found that $\text{E}$ is murdered. An inquiry is held, $\text{A, B, C, D}$ make the following statements.

Each makes two statements.

  • $\text{A says}$: I am innocent. $\text{B}$ was talking to $\text{E}$ when the train was passing through the tunnel.
  • $\text{B says}$: I am innocent. I was not talking to $\text{E}$ when the train was passing through the tunnel.
  • $\text{C says}$: I am innocent. $\text{D}$ committed the murder.
  • $\text{D says}$: I am innocent. One of the men committed the murder.
  • Out of the $8$ statements given above, $4$ are true, and $4$ are false.

Who is the murderer (Assume exactly one person is the murderer)?

  1. A
  2. B
  3. C
  4. D
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1 Answer

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Let the statements given by $\text{A}$ be $\text{A1, A2}$ in order. Similarly, let us denote $\text{Bi, Ci, Di, i} \in \{1, 2\}$ for other statements. Since one of $\text{A, B, C, D}$ did the murder, three among $\text{A1, B1, C1, D1}$ are true. Observe that the statements $\text{A2, B2}$ are inverse to each other. Therefore one of them is true. It follows that the four true statements are from $\text{A1, B1, C1, D1, A2, B2}$. Hence, we could conclude that $\text{C2, D2}$ are false. This implies, $\text{D}$ did not commit the murder, and none of the men committed the murder. Further, out of the two women $\text{B, D};$ since $\text{D}$ has not committed the murder, we conclude that $\text{B}$ is the murderer.

Detailed Video Solution: https://youtu.be/nclBhBmtz2g?t=3416

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