Wednesday, 24 October 2012

3 switches puzzle

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You have just walked into a dark two-storey house. There are three light switches on the first floor A, B and C. Only one of the switches works, and it turns on the light on the second floor.
You cannot see the second floor light from the first floor and there are no windows in the building, but you do have a flashlight.
You can only make one trip up the stairs, and you can only flip one switch at a time. How do you determine which of the three switches turns on the light on the second floor ?

Make sure that all light switches are in the "off" position. Flip the first switch A, and leave it on for about 10 minutes, or so. Then, turn switch A off. Flip the second switch B on, and go upstairs to the light bulb.
If the bulb is off, but warm, it is the A switch. If the bulb is on, it is the B switch. If the bulb is off and cool, it is the C switch that you haven't turned on yet. You have successfully determined which switch controls the light bulb, and you have gone up the stairs only once.

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