Personal Comment: Don't use math. Use everyday common sense.

Light Switch Enigma

What we know: There is a room containing three light bulbs each connected to a light switch outside the room. When you close the door, it is impossible to see inside. Your job is to figure out which light is turned on by which of the three switches outside of the room. You are allowed to go into the room only once.

Enigma: How do you know which bulb is turned on by which switch?

Solution: Turn on switch #1 and wait half an hour. Then turn it off and turn on switch #2. Go into the room. The lit bulb is connected to switch #2 and the warm bulb is connected to switch #1.

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