It is from an old math puzzle. I can't remember it well. But here is what I do.
"Two girls stand at an airport passageway.
One route leads to Quantas and the other to Branson's service.
One says you look nice and the other always tells the truth.
You are allowed only one question before continuing your journey (Sydney airport)."
I recall the solution being something like asking one girl what would the other say.