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Association for Computing Machinery

City of Lies or Truth (SPRING 2017)

Puzzle:

You are at an unmarked intersection . One way is the City of Lies and another way is the City of Truth. Citizens of the City of Lies always lie. Citizens of the City of Truth always tell the truth. A citizen of one of those cities (you don't know which) is at the intersection. What question could you ask to that citizen to find the way to the City of Truth?

Solution:

You ask "In which of those two directions do you live?". A citizen of the City of Lies will point to the City of Truth and a citizen of the City of Truth will point to the City of Truth.

(SOURCE: Math Is Fun)