This one’s a doozy in that there is a lot going on. The goal is simple, you need to escape a collapsing tunnel with a group of nine people (including yourself). It’s all the other variables that confuse things.
There are four potential exit tunnels but only one of them is actually the way out. Each tunnel is a 20 minute walk one way and another 20 minute walk back. You have an hour to get out. Ideally, you can send different groups into the tunnels and then meet back at the junction to know which way is the exit. But! Two of the people in your group may be lying (or may not be! you don’t know if they are or who they are), so what combination of people do you send down the people to guarantee your group’s exit?
Ted-Ed explains in the fun mind puzzle below. Consider the math.
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