The Three Switches
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The classic solution (which I won't give away) may not work too well with modern LED globes! CFLs maybe. Cheers, Peter
Software rusts. Simon Stephenson, ca 1994. So does this signature. me, 2012
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Downstairs in a house are three identical on-off switches. One of them controls the lamp in the attic. The puzzle is to work out which switch controls the lamp. The rules are as follows. You are allowed to manipulate the switches all you like, and then you are allowed a single trip to the attic. How do you do it? To clarify: When you are downstairs there is no way to see into the attic, or see reflected light from the attic. Once you have gone to the attic you are not allowed back to the switches. <edit>you can also assume original state of all switches to be off.</edit><hint>Otherwise it won't work</hint> Note: If you already know the answer, or reads the newspaper where I nicked the puzzle from, then please don’t write it up immediately in the comments! Give those who have never heard it before a chance to suffer, I mean enjoy finding the solution.
Wrong is evil and must be defeated. - Jeff Ello
Had this question at a job interview. It's a lateral thinking exercise. My experience with software development over 35 years is that lateral thinking is virtually never a part of the mental toolkit for software development. Oh, by the way, the solution requires that the bulb be an incandescent bulb. It won't work with an LED bulb or electroluminescent night light.
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Downstairs in a house are three identical on-off switches. One of them controls the lamp in the attic. The puzzle is to work out which switch controls the lamp. The rules are as follows. You are allowed to manipulate the switches all you like, and then you are allowed a single trip to the attic. How do you do it? To clarify: When you are downstairs there is no way to see into the attic, or see reflected light from the attic. Once you have gone to the attic you are not allowed back to the switches. <edit>you can also assume original state of all switches to be off.</edit><hint>Otherwise it won't work</hint> Note: If you already know the answer, or reads the newspaper where I nicked the puzzle from, then please don’t write it up immediately in the comments! Give those who have never heard it before a chance to suffer, I mean enjoy finding the solution.
Wrong is evil and must be defeated. - Jeff Ello
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Downstairs in a house are three identical on-off switches. One of them controls the lamp in the attic. The puzzle is to work out which switch controls the lamp. The rules are as follows. You are allowed to manipulate the switches all you like, and then you are allowed a single trip to the attic. How do you do it? To clarify: When you are downstairs there is no way to see into the attic, or see reflected light from the attic. Once you have gone to the attic you are not allowed back to the switches. <edit>you can also assume original state of all switches to be off.</edit><hint>Otherwise it won't work</hint> Note: If you already know the answer, or reads the newspaper where I nicked the puzzle from, then please don’t write it up immediately in the comments! Give those who have never heard it before a chance to suffer, I mean enjoy finding the solution.
Wrong is evil and must be defeated. - Jeff Ello
Here's my problem with the accepted solution -- How do you know that these aren't 3-way switches?? "One of them controls the lamp in the attic" -- which says nothing about whether the other two switches also control the lamp in the attic.
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Downstairs in a house are three identical on-off switches. One of them controls the lamp in the attic. The puzzle is to work out which switch controls the lamp. The rules are as follows. You are allowed to manipulate the switches all you like, and then you are allowed a single trip to the attic. How do you do it? To clarify: When you are downstairs there is no way to see into the attic, or see reflected light from the attic. Once you have gone to the attic you are not allowed back to the switches. <edit>you can also assume original state of all switches to be off.</edit><hint>Otherwise it won't work</hint> Note: If you already know the answer, or reads the newspaper where I nicked the puzzle from, then please don’t write it up immediately in the comments! Give those who have never heard it before a chance to suffer, I mean enjoy finding the solution.
Wrong is evil and must be defeated. - Jeff Ello
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Downstairs in a house are three identical on-off switches. One of them controls the lamp in the attic. The puzzle is to work out which switch controls the lamp. The rules are as follows. You are allowed to manipulate the switches all you like, and then you are allowed a single trip to the attic. How do you do it? To clarify: When you are downstairs there is no way to see into the attic, or see reflected light from the attic. Once you have gone to the attic you are not allowed back to the switches. <edit>you can also assume original state of all switches to be off.</edit><hint>Otherwise it won't work</hint> Note: If you already know the answer, or reads the newspaper where I nicked the puzzle from, then please don’t write it up immediately in the comments! Give those who have never heard it before a chance to suffer, I mean enjoy finding the solution.
Wrong is evil and must be defeated. - Jeff Ello
Its so simple.. you turn ON ALL switches
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Downstairs in a house are three identical on-off switches. One of them controls the lamp in the attic. The puzzle is to work out which switch controls the lamp. The rules are as follows. You are allowed to manipulate the switches all you like, and then you are allowed a single trip to the attic. How do you do it? To clarify: When you are downstairs there is no way to see into the attic, or see reflected light from the attic. Once you have gone to the attic you are not allowed back to the switches. <edit>you can also assume original state of all switches to be off.</edit><hint>Otherwise it won't work</hint> Note: If you already know the answer, or reads the newspaper where I nicked the puzzle from, then please don’t write it up immediately in the comments! Give those who have never heard it before a chance to suffer, I mean enjoy finding the solution.
Wrong is evil and must be defeated. - Jeff Ello
I'd say I'll use logic gates for this one. Not sure if it will be valid, don't care either if it is or not. :laugh: :laugh: Say we are talking about OR gates, we can have a single switch be turned differently from the other two switches. We will then have 1+0+0=1 or 1+1+0=1. Then the lamp is on.
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Downstairs in a house are three identical on-off switches. One of them controls the lamp in the attic. The puzzle is to work out which switch controls the lamp. The rules are as follows. You are allowed to manipulate the switches all you like, and then you are allowed a single trip to the attic. How do you do it? To clarify: When you are downstairs there is no way to see into the attic, or see reflected light from the attic. Once you have gone to the attic you are not allowed back to the switches. <edit>you can also assume original state of all switches to be off.</edit><hint>Otherwise it won't work</hint> Note: If you already know the answer, or reads the newspaper where I nicked the puzzle from, then please don’t write it up immediately in the comments! Give those who have never heard it before a chance to suffer, I mean enjoy finding the solution.
Wrong is evil and must be defeated. - Jeff Ello
Assuming the bulb is incandescent and the switches are 1 -2 - 3 turn on switches 1 and 3 - wait 10 minutes - turn off switch three go to the attic - if the light is on, switch one activates the lamp if the light is off, feel the bulb if it is warm, switch 3 activates the lamp if it is cold, switch 2 activates the lamp. SushiSlayer
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Downstairs in a house are three identical on-off switches. One of them controls the lamp in the attic. The puzzle is to work out which switch controls the lamp. The rules are as follows. You are allowed to manipulate the switches all you like, and then you are allowed a single trip to the attic. How do you do it? To clarify: When you are downstairs there is no way to see into the attic, or see reflected light from the attic. Once you have gone to the attic you are not allowed back to the switches. <edit>you can also assume original state of all switches to be off.</edit><hint>Otherwise it won't work</hint> Note: If you already know the answer, or reads the newspaper where I nicked the puzzle from, then please don’t write it up immediately in the comments! Give those who have never heard it before a chance to suffer, I mean enjoy finding the solution.
Wrong is evil and must be defeated. - Jeff Ello
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Downstairs in a house are three identical on-off switches. One of them controls the lamp in the attic. The puzzle is to work out which switch controls the lamp. The rules are as follows. You are allowed to manipulate the switches all you like, and then you are allowed a single trip to the attic. How do you do it? To clarify: When you are downstairs there is no way to see into the attic, or see reflected light from the attic. Once you have gone to the attic you are not allowed back to the switches. <edit>you can also assume original state of all switches to be off.</edit><hint>Otherwise it won't work</hint> Note: If you already know the answer, or reads the newspaper where I nicked the puzzle from, then please don’t write it up immediately in the comments! Give those who have never heard it before a chance to suffer, I mean enjoy finding the solution.
Wrong is evil and must be defeated. - Jeff Ello
I haven't looked at the other posts yet, so I'm throwing my solution in before I go reading. Turn on the first switch. Wait a while. Turn off the first switch. Turn on the second switch. Go upstairs. If the light is on, it's the second switch. If the light off, feel the bulb. If it's hot, it's the first switch, otherwise it's the third switch.
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Downstairs in a house are three identical on-off switches. One of them controls the lamp in the attic. The puzzle is to work out which switch controls the lamp. The rules are as follows. You are allowed to manipulate the switches all you like, and then you are allowed a single trip to the attic. How do you do it? To clarify: When you are downstairs there is no way to see into the attic, or see reflected light from the attic. Once you have gone to the attic you are not allowed back to the switches. <edit>you can also assume original state of all switches to be off.</edit><hint>Otherwise it won't work</hint> Note: If you already know the answer, or reads the newspaper where I nicked the puzzle from, then please don’t write it up immediately in the comments! Give those who have never heard it before a chance to suffer, I mean enjoy finding the solution.
Wrong is evil and must be defeated. - Jeff Ello
I attempted to prove the solution is viable but in the attempt I broke my ankle stumbling over all the junk in the pitch black attic as I tried to get to the light. :(( >>> Mental note: Only attempt this during the daytime and only for attics with windows <<< :laugh:
- Kelly