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Answer to Riddle #7: Three Light Bulbs & Switches in a Room
A windowless room contains
three identical light fixtures, each containing an identical light bulb or light globe.
Each light is connected to one of three switches outside of the room.
Each bulb is switched off at present. You are outside the room, and the
door is closed. You have one , and only one, opportunity to flip any of
the external switches. After this, you can go into the room and look at
the lights, but you may not touch the switches again. How can you tell
which switch goes to which light?
I got some help with this one, from
Eric Cunningham
of Virginia University, Zachary
Sung and Julian Lepinski all who visited me from the charmingly entertaining
fark.com and Fred Piceno who emailed me...
Before reading the answer can I interest you in a clue?
The consensus of opinion seems to be either this:
Switch on switches 1 & 2, wait a minute and switch off number 2.
Enter the room. Whichever bulb is on is wired to switch 1, whichever is off and hot is wired to switch number 2, and the third is wired to switch 3.
OR this:
Switch on switch number 1, wait a few
minutes and switch on switch number 2.
Enter the room and which ever bulb is on and is hot is connected to switch 1, whichever is on and cold is
connected to switch number 2, and the remaining bulb is connected to switch number 3.
Either solution is logically equally valid. The first may be more practical.
This puzzle is unusual for this site in that it is not completely pure, it's pragmatic rather than pure logic. But it is a very common question and that earns it it's place.
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