Check the log

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

Lockout both rooms and log a job with maintenance.

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?

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.

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.

"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.

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.

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:

• 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).

• 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.

• 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.

• 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.

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.

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

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.

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.

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.

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.

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.

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.

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.