Sorry, Andrewas. You're close, but...
Quote:
Ask one of the demons "What did you say to the last adventurer that asked which door was safe to enter?"
The lying demon will have lied to the adventurer, and will lie about that, therefore telling you the correct door
The truthful demon will tell you the truth about telling the adventurer the correct door, and will therefore also tell you the correct door.
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will fail if (a) you're the first adventurer in the room, or (b) the last adventurer asked the other demon, in which case this demon said nothing to that last adventurer.
There is a straightforward answer. You're almost there.