[Mathematical logic (miscellaneous).]

Cowboy Joe was sentenced to death in the electric chair. He knows that one of the two electric chairs in a special chamber is faulty. In addition, Joe knows that if he sits on this faulty chair, the execution will not be repeated and he will be pardoned. He also knows that the guard, who is guarding the chairs, answers truthfully every other day and lies every other day. The condemned man is allowed to ask the guard exactly one question, after which he must choose which electric chair to sit on. What question can Joe ask the guard to definitely find out which chair is faulty?
Recall problems $140,148,149$.

## Solution

Joe can, for example, ask: "What would you have answered me yesterday to the question which chair is broken?"

And regardless of whether the guard is telling the truth today or lying, the answer to this question will always be incorrect. Interestingly, if Joe uses "the day before yesterday" instead of "yesterday" in this question, i.e., asks: "What would you have answered me the day before yesterday to the question which chair is broken?", the answer will always be truthful, regardless of whether the guard is telling the truth today or was telling the truth yesterday.

## Answer

"What would you have answered me yesterday to the question which chair is broken?"

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