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This question already has an answer here:

There is an old house with an upstairs room and a downstairs room. the upstairs room contains 3 light bulbs and the downstairs room contains 3 switches. Each switch is connected to one light bulb.

A man is in the downstairs room and needs to know which switch belongs to which light bulb, however the man can only run up the stairs once to check what happened when turning on/off the switches.

How does the man know which switch belongs to witch lightbulb?

Btw the upstairs room/lights can not be seen from downstairs

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marked as duplicate by Rubio Mar 22 '18 at 10:06

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the poor man doesn't live in a conveniant house, but at least he can know witch switch correspond to witch lightbulb this way:

The man switch on the first switch, wait 10 minute and switch it off. this would made the lightbulb corresponding to first switch hot. Then, he switch on the second switch. The lighting lightbulb cill correspond to the second switch, and the cold-off one to the third

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