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10. In your cellar, there are three light switches in the ‘off' position. Each switch controls one of three light bulbs on the floor above it. You may move any one of the switches but you may only go upstairs to inspect the bulbs once. How can you determine the switch for each bulb with only one inspection?

Answer: Turn the first switch on for 5 minutes, then turn it off. Turn the second switch on, and leave it on. Don't touch the third switch. When you go upstairs, the bulb that is off, but warm is connected to the first switch, the bulb that is on is connected to the second, and the bulb that is off and cold is connected to the third.

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