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This question already has an answer here:

The following puzzle being very much recreational for me, I couldn't resist myself from sharing it with my fellow MSE user friends. Let's have a look at it.

You are the most eligible bachelorette in the kingdom, and as such the King has invited you to his castle so that you may choose one of his three sons to marry. The eldest prince is honest and always tells the truth. The youngest prince is dishonest and always lies. The middle prince is mischievous and tells the truth sometimes and lies the rest of the time.

As you will be forever married to one of the princes, you want to marry the eldest (truth-teller) or the youngest (liar) because at least you know where you stand with them.

The problem is that you cannot tell which brother is which just by their appearance, and the King will only grant you ONE yes or no question which you may only address to ONE of the brothers. What yes or no question can you ask which will ensure you do not marry the middle brother?

Though it wasn't given in the original problem let's impose the condition that "No answer" isn't a possible answer.

Note: this problem is also called the Bachelor Problem, with genders reversed (explicitly mentioned so that (SE) searches on either name will succeed).

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merged by robjohn Feb 11 '15 at 16:15

This question was merged with The Three Princesses (distinguishing truth-teller with 1 question) because it is an exact duplicate of that question.

  • $\begingroup$ I strongly disagree with the assertion that the question linked as this question's duplicate has an answer. It does not have an answer that meets the requirement here of not knowing which sibling is which, whereas this question has very well presented answers that do cope with that condition. $\endgroup$ – Joffan Feb 6 '15 at 1:56
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    $\begingroup$ @Joffan: The difference you refer to is not relevant to the puzzle. In the duplicate, the relative ages are known, not the truth telling status. This makes it easier to refer to them, but doesn't affect the answer. Instead of oldest, middle, and youngest, here you have hey you, that one, and the other one. $\endgroup$ – Jonas Meyer Feb 6 '15 at 3:46
  • $\begingroup$ @JonasMeyer Nevertheless, the answer there is not actually an answer. And part of the criterion for marking a question as duplicate is that there is already an answer. $\endgroup$ – Joffan Feb 6 '15 at 4:31