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Lora has an old house in the countryside, which includes a ground floor and an upper floor. She decides one day to install electricity. An electrician comes and installs three switches on the ground floor. Having only one bulb which she installed on the ceiling of her room located on the upper floor, she decides to determine which switch is connected to this bulb, knowing that she does not have any means of seeing it turn on or off from the ground floor, but that she can turn the three switches on or off whenever she wants to. Moreover, Lora is really old and tired, and her goal is to go up only once to the upper floor.



What trick will Lora use to find the right switch without climbing the staircase more than once?
Created 2006 - 07 - 22

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