Post:
You have three switches in one room and a single light bulb in another room. You are allowed to visit the room with the light bulb only once. How do you figure out which switch controls the bulb? Write your answer in the comments before looking at other answers.
Comment:
If this were an interview question, the correct response would be "Do you have any relevant questions for me? Because have a long list of things that more deserving of my precious time than to think about this!
Even knowing the “correct answer” to this riddle for as along as I remember, I don’t think it is right. For someone looking for how to handle this in an interview, I’d go with this:
I will fetch a friend or colleague to look at the bulb as I test the switches because:
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It is by far the most obvious solution that literally everyone faced with this problem actually would use. It is easy to understand and will be easy to explain to others (if you, e.g. need to present or document what you did).
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It is also a better solution: it is by far more robust against a large number of failure modes: e.g., if it turns out you are testing the wrong switch, the bulb is broken, more than one switch turn on the light, etc.
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It scales better: the same solution trivially extends to N number of lightbulbs controlled by M number of switches; and at large N it will save time not having to reach each bulb.
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It gives the opportunity to interact positively with a friend/coworker. Helping each other out with small necessary tasks builds team cohesion and work environment, and thus lowers the barrier for further collaboration, making us a more effective team in the longer run.
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What bothers me about this specific question, apart from it being dated, is that it breaks the rules of these kind of riddles. They’re implied to be in a sort of frictionless sphere universe, the whole preposition is silly except as an abstract puzzle. To then rely on the physical properties of real lamps is cheating. You’re supposed to ignore all the real-world aspects of the setting except that one.
Agreed, it presents as an abstract logic puzzle, but then gives a very concrete answer. It’s like presenting the trolly problem to someone, and when they give one of the two expected answers saying “no, stupid, you run ahead and untie the victims before the trolly reaches them.”
It’s compounded by the fact that the proposed physical solution isn’t even very reliable, as lots of people in this thread have said. If we’re stepping outside of the logic puzzle constraints, why not just leave the door to the room open? Or have someone stand inside and shout when the light turns on? Or ask someone who knows these switches? Or any number of boring non-brain teaser solutions.
For those that want the actual answer:
Tap for spoiler
You turn on the first switch for a minute or two, turn it off, and turn on the second switch. If the bulb is on, it’s obviously the second switch. If the bulb is off and warm, it’s the first switch. If it’s cold, it’s the third switch.
This assumes several things to be true, which might not be true:
- power is available/the upstream circuit is on (always a bad assumption to make)
- the bulb is an incandescent type that will generate an appreciable amount of heat in a short amount of time
- the bulb was in the off state before you changed the position of any switches, and has been off long enough to be cold
- the bulb is connected to any of the switches
- the bulb is connected to only one of the switches (parallel circuits are a thing, as are multi-switch lighting circuits)
If any of the above is not true, the conclusion is invalid.
I’ll go one further:
- Assumes the bulb is in reach. When I read the problem I assumed the bulb was in a ceiling fixture out of reach. Nowhere in the text description did it specify the physical location, except “in the other room”.
The biggest flaw is that it assumes you’ll add conditions you’re not explicitly told are allowed. Many, many problems in school would be trivial if changing the terms beyond what’s stated was allowed.
This is often exactly what the interview question is testing. Many of these questions are not about the solution but about how the applicant approaches problems
Yet they never explicitly state you’re allowed to make convenient assumptions. If the bulb was out of hand’s reach the problem would be unsolvable.
Assuming the electrician that wired the switches is in the room would be even a more out-of-the-box solution.
As I said, they care about how you think. Do you ask all these questions?
if I were given this interview question I would immediately start asking questions: Do I have my phone? Can I bring any objects into the room? Do I know the construction of the light? How far from the room is the light switch panel?
Asking “what are the limitations and conditions of this situation” is literally the thing they want to see. That’s my entire point.
The official answer to this riddle is turn switch 1 on for a minute or so, switch it off then switch 2 on. if the bulb is hot but dark, its 1, if it’s lit it’s 2 and if it’s out and cold its 3.
the adult answer is why do I have only one chance to walk in the room?
8 lightswitch states. Smack em all on, and smack em all off. If there’s no change, that’s a bad lightswitch
this is the classic answer but it also fails pure logic because the question only implies one of them actually works, and even then, it’s only one of them. the truth is any number of them could work, or a specific combination, or a number of combinations, or it might be none. the bulb itself to could be busted. my point is not to be an uncooperative asshole but that a logic puzzle that relies on real world properties should cover its bases.
