For those that want the actual answer:

::: spoiler 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. :::

I really hate these awful "puzzles". They only work by the asker intentionally withholding what, if any, constraints exist in the problem space leaving it totally vague, but of course there ARE secret constraints revealed if you violate them with your answer.

Me: "I do it without flipping any switches. I just ask the lightswitches which one controls the light, and they tell me."

Interviewer: "That's not allowed."

Me: "Well what exactly is allowed? Can I pull the cables out of the wall and see which connects to the bulb? Oh, I bet that's not allowed. How about I open my smart home app and just check which of the smart switches is labeled for it? Oh, I bet it's not a smart switch so I can't do that either? Oh, then the bulb has a chime that boops when it comes on, so I just listen for the boop. Oh that's not allowed either? Wait wait wait, the walls are glass, so I just watch to see when the bulb comes on when I flick the switches."

Even the canonical answer makes a dumb assumption. Ordinary LED bulbs don't get hot.

Ha! Easy! Go in the other room and take a picture of the bulb. Now go back to the switches and flip each one in order, while looking at the picture. When the picture of the bulb shows it lit up, that’s the switch.

Answer:

::: spoiler Tap for spoiler Flip two switches and check the bulb. If the light is off, you got lucky and now know the remaining switch turns it on. If the light is on, you now know one switch that won’t turn it on. Return to the room and finger your asshole. You’re now having more fun than solving a logic puzzle. :::

Ok. The classic answer is "turn on the first switch for five minutes. Then turn switch 1 back off, turn on the second switch and go in the room immediately. If the light is hot, it's controlled by switch 1; if it's on, it's controlled by switch 2; if it's off and cold it's controlled by switch 3."

Except that a light bulb in 2025 is very likely to be an LED bulb, so it wouldn't actually get hot. At least not hot enough to feel even a few moments later. And in a corporate setting (this is classically an interview question), the switch has been more likely to control a fluorescent tube, which can get hot, but typically not as quickly as an incandescent one.

My answer, if I were in an interview, would be to ask questions (Chesterton's Fence).

• First of all, why do we have the one-visit limit? Is this a prod light bulb? We need a dev light bulb environment, with the bulbs and switches in the same room. (While we're making new environments, let's get a QA and regression environment, too. Maybe a fallback environment, depending on SLAs.)

• Second, what might the other switches do? What's the downside to just turning them all on? If that's not known, why not? What is the risk? For that matter, do we know that only one switch needs to be turned on to turn on the light, or is it possible that the switches represent some sort of 3-bit binary encoding?

• Third, why were the switches designed this way? Can they be redesigned to provide better feedback? Or simplified to a single switch? If not, better documentation (labeling) is a must.

• Fourth, we need to reduce the length of the feedback loop. A five minute test and then physically going to touch the bulb is way too long. Let's look into moving the switches or the light in our dev environment so that the light can be seen from the switches.

The answer isn’t intuitive anymore now that lightbulbs don’t always get hot 🥲

I don't understand. You don't need to visit a room to know whether the light is on in it.

Unlabelled switches controlling lights in another room isn't Workplace Health and Safety approved.

Lockout both rooms and log a job with maintenance.

"First, I would get a label maker and ask a coworker to assist me. Then, we'd work together to quickly figure out what each switch does, and then label them accordingly. In a business of this size and reputation, documenting your work and synergistic teamwork are foundational to value and growth."

Then, reject whatever offer they send and say that it's because they showed you a workplace culture that enabled middle management to test employees with busywork instead of minding their own business or solving their own damn trivial problems.

Go into the room and unscrew the bulb. You can now truthfully say that no switch affects the bulb’s condition, without messing with a bunch of switches whose function you don’t understand. You even know for a fact that the lack of bulb won’t cause a problem down the line, since the room is apparently no longer accessible.

Based on the provided information, there are some switches of unspecified type in one room and a light bulb of unspecified type in another room. There is no power source, nor do we know if there is even wiring between the switches and the bulb. For all we know, the switches and the bulb are still in their product packaging waiting to be installed by an electrician.

The bulb is not controlled by any of the switches in any meaningful manner.

Also, per the problem specification, I am allowed to visit the room with the light bulb only once. I am not allowed to visit the room with the switches, or operate the switches.

The comment in the original image is the most rational possible answer to such an exercise. Poorly stated problems are a waste of time.

*Edit: You know what, scratch all that, none of it really matters.

I'm not messing with an unknown electrical circuit without seeing the circuit diagram and verifying any relevant lockout/tagout. People die from that shit.

Ok, what do we know. We know the bulb isn't screwed into anything. We also know the switches are in the "on" position but the bulb is not illuminated. From that, we can conclude that the switches do not control the bulb at all, or the bulb is somehow wirelessly controlled by the switches. We bring the bulb through and throw the switches one by one, see what happens.

This is either a really clever test of your problem solving and neccessary-information-extraction skills. Or a really dumb one with loads of asumptions and artifical restrictions and based on outdated data (comments hint to the lightbulb getting hot).

Remove the switches put a microcontroller like esp32, connected via wifi to an app on your phone. Go to the other room and see which switch switches on the bulb.

If there is no wifi, why the hell do you want a programmer. I can't work without internet.

Take the cover off, flip all three switches. Whichever terminal shocks you is completing the circuit for the light.

My burn-the-house-down take on this: very slowly flip each switch on and listen for arcing. Works fine assuming the other two switches aren't connected to anything.

I'll look through the door.

Or, set up a webcam to see when the light is on.

If this isn't allowed somehow, I'll tell the building management to consider rewiring this absolutely cursed light switch situation ASAP because it's gotten so bad that it's being used as a brainteaser by the recruiting department

This question becomes more a test of age as time goes. I've been asked this question even after the movement towards all-LEDs.

This question is also stupid, both because it has a correct question and because almost certainly some people have advantages over others that have nothing to do with the actual job.

20+ years ago? Sure, this was a somewhat viable question. But now? It's incredibly messy.

Over my years, I've asked dozens of very, very smart people from all kinds of walks of life, extremely smart to seemingly dumb as hell - nobody has ever gotten it right.

Probably the only thing this question is good for is seeing how an applicant does when faced with a diplomatic situation and a really dumb interviewer.

I'm super curious what the people who unironically ask this question think they're testing.

Building codes, at least the ones I am aware of, require the light switch inside the room with the light next to the door, similar to how nearly every room you have ever been in. (Everyone knows of exceptions.) This means either corners have been cut, at those switches should control things within the room with the switches.

As the interviewer if attention to detail and following build codes and specifications is important at this company. Is there a culture of safety, or are corners cut that put my life at risk.

After reading the incandescent bulb solution, and problems regarding touching the bulb, i would switch first switch on for a appreciably long time, such that bulb has hit maximum luminousity (they heat up as they run, the hotter they get, the brighter they are), then turn switch off, and turn second switch on and quicky run to other room. we are trying to observe change in luminousity as time elapses. if it reduces, it was first (we ran it for a long time, there would be some residual glow, from my irl observations from when i was small suggest roughly 1 min period where i can still tell, but bulb wattage, contrat with background and distance matter). if increasing or max luminous, then second, if nothing then third.

but it was a stupid question. my naive guess was it can not be done, because with just 1 binary observation, you can not tell from 3 switches (you need atleast 2, which the solution assumes as temp and light state, i substitute heat with light state in transition). but still stupid. my natural assumption was leds, even when i head incandascent bulbs in my house somewhere for nearly half of my life. it is also stupid, because when you allow me to do something i was mentioned in question to do, i could just bend my way to do anything. like punch/drill through wall, or hack surveillance systems, or just pull out my handy multimeter that i always have on me, open switch box and see which switch is live, which is dead, or see voltage/current/wattage change across the loop, or measure resistance and guess what thing is there, or like blackmail the interviewer to extract the answer.

I would say, I do enjoy riddles, so this will be fun. But I am concerned that if you think my skill at riddles is critical, that it may mean your management has gotten used to not fully thinking through the objectives they give and how those objectives interact with the existing systems or other objectives. That would result in the kind of product that looks like the right hand doesn't know what the left is doing. If that is your reasoning for the question, how is the company countering it to create a coherent product.

And the reason I might say this is tgat in my experience, companies who ask such questions aren't the kind I want to work for.

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.

Here's my answer that works with any kind of lightbulb.

Flip switch 1 on, switch 2 off, and get switch 3 stuck in a halfway point which I've done on both lever switches and flat switches.

If it's on it's switch 1, if it's off it's switch 2, if it's flickering or dimmed it's switch 3 and you should probably turn it off to stop damaging the relay.

Grab the bulb and bring into the room with you and replace it into a light fixture in your switch room. See which switch controls it. If this doesn't work, retrieve the bulb and answer with "currently none of the switches control it."

Literally ask someone which switch it is. Then ask them what idiot wired them up that way

I'd walk into the other room first and drop a mirror in the hallway on my way back so I could see lmao. My employer wouldn't want me touching a hot bulb since that might be a workplace hazard they'd be liable for after training me with stupid riddles.

Impossible even if you know if the light is on or off to start with. Even then, there are 2 possible outcomes which means the solution space halves on each test. 3 divided by 2 is greater than 1 (1.5) so we cannot figure it out in a single test.

That's my recollection of how to solve these from computer science. The classic one is 8 coins and figuring out which one weighs a different amount (and you don't know if it is more or less). You have a scale that tells you which side is heavier (or equal) but it doesn't give readouts (as in it doesn't say a side is X pounds/grams). With only three uses of the scale, how can you find the fake coin? I'm not going to go into the process in depth but because you have THREE outcomes (left heavier, equal, and right heavier) you reduce the solution space (which of the 8 coins is the bad one) by a THIRD each test. The number 8 sort of lures into thinking powers of 2. You can actually do it with 9 coins in 3 tests.

Some of the details of my explanation may be wrong, it's been over a decade since I took that class in college lol. It was my worst professor (while different story lol) but I distinctly remember him talking about this. He had a very thick accent, some form of eastern European or Russian, I'm not really sure what exactly. But he gave us that problem as homework or something or maybe just to think about. And he'd ask us to explain how we'd do it. Whenever someone began to describe something doing like test 4, 2, etc instead of the correct way (which involves using coins you already tested) he'd say "YOU'RE DOOMED!" Then someone else would try, and when they got to a way that wouldn't work "YOU'RE DOOMED!" It was hilarious. Very memorable.

You can get a 66% chance of being right if you turn on a switch and check the room. If the light is off you guess between the other two. For example, you are going to test switch 1 and if it is off you guess switch 2.

Reminds me of the Monty Hall problem except in that problem the game show host has an action to take which effects the odds.

If the installation of the circuit was done correctly in the first place, all 3 switches will turn the light on and off.

If they do not, there is a problem and it needs to be fixed. If you don't fix the issue, you have a major underlying problem in your company. And you are not worth my time.

It depends on what type of person designed the circuit and what type of person you are.

Ergonomics: The switch closest to the door first, then mid, then far, figuring the unknown user would click the switch closest, a skilled electrician would start there. However, it's not unreasonable for the electrician to ask the owner, so this is a hit-or-miss approach.

Installation efficiency: The installer refused to mark any of the lines and instead hooked them up at random, flip in any order, when you find the right one, return the others to the original state.

time efficiency: the energy cost to flip all three switches is minimal and you're only going in once, flip all three at the same time. you've done maximum effort and maximum time savings.

Error reduction, binary counter, all combinations tested in case of chained switching

Debugging: binary counter, followed by checking the lightbulb, possibly swapping for another if one is nearby, checking all the other switches near the room, breakers, power to the structure, and asking an occupant for assistance as a last resort.

Disaster recovery: locate a flashlight or use your phone's torch/flashlight function.

Ahh crap, other room.

1. ask an occupant

2. shove a penny in the socket behind the light bulb and listen for a breaker to pop

3. turn all three on

4. slide your cell phone under the door with video recording on, stomp on the floor hard every time you flip a switch

5. turn all the switches through a binary counter looking for one that seems to do nothing.

I’ve walked out of interviews that had these popular puzzle questions in the 00s. The company you’re interviewing for is not testing you for your job, it wants a corporate drone that is ok with bureaucracy and can navigate the red tape they’ve put in place.

Really a waste of time, but if I run into this at my age now I ask if they can tell me how their company is making something for the betterment of human kind.

Thinking skills, why would those be needed?

Disassemble the switch and look what's connected.

turn them all on, problem solved

Knowing full well this would be coming from a FAANG company, a funnier answer would be to replace the switch with the equivalent smarthome switch, and then spend the next 20 minutes explaining their uttery stupid network pathway from your phone, through the cloud, back to your device to turn on a lightbulb.