There are three light bulbs on the tenth floor of a building and there are three switches on the ground floor which are used to turn on the light bulbs (one switch turns on one bulb).
A man is on the ground floor and he knows that the three light bulbs on the tenth floor are all turned off.
How can he just goes up to the tenth floor only ONCE to determine which switches control which light bulbs?
First, turn on one of the switch for a long time, e.g. 5 minutes, then switch it off, then turn on another switch, then immediately go to the tenth floor.
On the tenth floor, see which light bulb is turned on. that light bulb corresponds to the switch which is turned on at last.
Sense the two light bulb which is not turned on, see which one is hotter, the one which is hotter corresponds to the switch which has been turned on for a long time but turned off subsequently.
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