Brain Teaser

There are two interconnecting rooms. In the first room that you are currently in there are three light switches. In the second room there is a light bulb. One of the three switches is connected to the light bulb and the other two are not connected to anything. You may pass from the first to the second room just once. When you arrive in the second room you must state accurately which switch controls the light bulb. It is possible to be 100% accurate. There is no way to be able to see into both rooms at the same time and there no way to operate the switches while looking at the light bulb. There is also no windows in either room (either in the walls or in the roof).
Comments:

This teaser used to apply some years ago, but what if the lights were LEDs? ...

To those still working on this...it makes a difference.

Posted by Wes W. on July 31, 2007 at 11:43 AM BST #

I can sense the heat :)

Posted by daisychain on July 31, 2007 at 12:11 PM BST #

With traditional bulbs this is pretty simple but it doesn't work with my CFL bulbs. Try this puzzle in a few years time and nobody in the Western world will have a clue. There are some ways of doing this that are rather outside the spirit of the question. Measure the current drain on the circuit or short out the wires and very quickly determine which switch is live. Solution, incidentally, assumes that the switch off position is known. If the light is on at the start then the solution looks a bit dubious.

Posted by willow on July 31, 2007 at 03:24 PM BST #

In the same style, with incandescence lights, you have the following problem:
the first room has 4 switches, and the second room has 4 light bulbs.
Each switch is connected to one and only one of the 4 lights, but you don't know the mapping between switches and lights.
Once again, you can only go from one room to the other once.
How do you establish the eact mapping between switches and lights?

A good test is to see if a person who couldn't solve the first problem is able to solve the second one ("mine") after getting the solution to the first one.
If they still can't, then they've got a real problem with ... problem solving.

Posted by Dominique Grabas on August 10, 2007 at 06:57 AM BST #

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