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Photo: DER SPIEGEL
A woman has landed on an island where there are two groups of people.
Some always lie, others always tell the truth.
There is no external characteristic by which one can distinguish the people from the two groups from one another.
You should be familiar with this setting.
In a café, the woman meets a man who she does not know which group he belongs to.
She can ask him a question to find out.
However, the question must meet one condition: the woman must not know the correct answer, i.e. the one corresponding to the truth, when she asks her question.
The obvious question of what 1 + 1 is has now been settled.
The woman thinks about it for a moment, starts to grin and then says: "I know what I'm asking."
Do you know it too?
The woman takes a stack of playing cards from her pocket, pulls one out of it without looking at it, and holds it up to the man.
"Is that an ace?" She asks him.
When she hears the answer, she turns the card over and immediately sees whether the man lied or not.
If you missed a puzzle from the past few weeks, here are the ten most recent episodes:
Enclosed by six circles
x, y unsolved
Floor plan with gaps
Wrong track
Seven in one go
Hundred square numbers
On the brink
Secret access code
Who lies?
Who is telling the truth
How does the sequence of numbers continue?