Minute Mystery: The Two Doors

Bite-sized brain teaser to solve in a minute or less.

 

 

The Two Doors

Category: Classical Logic / Deductive Reasoning

You come across two doors:

• One leads to freedom

• One leads to danger

The doors are protected by two guards:

• One always tells the truth

• One always lies

 

You may ask one of the guards just one question. What do you ask?

 

 

Solution

Ask either guard: “Which door would the other guard say leads to freedom?” Then choose the opposite door. Why? The truth-teller reports liar’s lie. The liar lies about truth-teller’s answer. Both point to the wrong door. Answer: Ask what the other would say - choose the opposite.