Its a classic riddle, so their’s every chance she already knew the solution.

But it goes like this:

You *know * one guard will lie, one tells the truth. The trick is to phrase the question such that you can get the same response out of both, in such a manner it highlights the liar.

When you ask a guard “what will the other guard tell me is the safe passage?”, the lair will lie, and tell that the truthful guard would say the unsafe passage.

The truthful guard will truthfully report that the liar would lie and say the unsafe passage, so whichever way round it goes, you know the safe passage is the one they DON’T say to go down.

Make sense?

She asks one of them, is he going to tell me this door leads to the castle.

If answered by the liar, if the door is deadly, then the other guard will say no, but since he lies, he would answer yes.

And if the door leads to the castle, then the real answer is yes, but since he lies, he would say no.

If answered by the truth teller, if the door is deadly, the liar will lie, so he would answer yes. If the door leads to the castle, the liar will lie and the answer is no.

So if the answer is yes then the door being asked is deadly, otherwise the door leads to the castle.

It’s exactly like she explained. One tells the truth and the other always lies. If she asked the liar what the right path would be, they would give the wrong answer. The truth-teller would know that so they’d say that the liar would give the wrong answer.

If she asked the truth-teller what the right path is, they’d give the right answer. If she asked the liar what the truth-teller’s answer would be, they’d lie and give the wrong answer.

Either way, she can safely say that the correct path is the opposite of what they said.

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