Can you solve the dark room coin riddle?

Originally published at:

Try chocolate coins instead, will be much easier that way.


Don’t you just have to bite each coin? as the gold side will be softer while the silver side will be rock hard.


Do you want me to tell the solution?

OK, I’ll spoilerize it, hopefully.

Take 20 coins and flip them each over to make a new pile. You’re done.


Gold is hard. People used to bite gold to check that it was hard. Inferior metal alloys that look like gold were often soft. So if it made a mark when you bit it, it wasn’t gold.


Medievalist’s solution works because:

[spoiler]If you happen to take a gold-side-up coin and flip it into your pile, you now have a silver-side-up coin, balancing one in the original pile. (Do this 20 times and obviously you’re good: there are 20 silver-side-up coins left untouched in the original pile.)

What if you happen to take a silver-side-up coin, won’t you then be missing one as in the new pile it will become gold-side-up? Well yes but it doesn’t matter because the original pile will also have one fewer silver-side-up coin in it (namely the one you just flipped).

I agree, surprisingly simple![/spoiler]


I was thinking the specific heats might be sufficiently different that you could rub the coins and determine if one side was noticeably warmer than the other. But biting makes much more sense.

Alternatively: take two coins to the blue-eyed person and two coins to the brown-eyed person and wait to see which of them takes off in an airplane after the treadmill starts running. Ask the remaining person if the other one was lying; then, take advantage of the momentary distraction by attacking his vulnerable eyes. Or something.


Although the animated story doesn’t really make this rule clear, the answer cannot be anything about biting the gold, or heating it, or anything like that. It’s made clear in the posted screenshot that “There is no way to tell which side of any particular coin is facing up.” Even if you dispute that on logical grounds, the riddle is making it clear that differentiating the coins is not a component of the intended answer.


You need carriage returns after [spoiler] and before [\spoiler], apparently, for it to work.

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Damn, I was close. I would have flipped one-half the coins and applied the Central Limit Theorem. Then, if you assume the pile of coins is infinite, you’re done!


I seriously do not know where you are getting your info from…by all means just quick google is gold hard or soft.

While it is classified as a hard metal, it is actually soft and rather malleable. Which is why so many things were made from it…it is malleable.

While I can concede if the coin was merely painted gold/silver or something like that biting it wouldn’t work. The reality is if it was some ancient set of unrefined gold and silver coins, the gold would be softer. This is science, you cannot make up alternative facts.

gold=soft. end of story.


Pick up the coins and leave.


But the room is dark and the doors feel exactly like walls. Or something like that.


Use the flashlight on your phone. duh.


You mean, like a tree?


It turned out there were no coins in the dark room. The last person you tested must have stolen them! I know a guy who could get you a deal on an IR security system.

Oh these coins? I brought these coins with me from home.


To be honest, I got it from QI, so maybe it’s wrong. I’ve never tried biting coins. Have you?

I’ve bitten various gold quality items before.

Again. Science. Facts. The purer the gold is the softer it is.

These are facts. Argue away until your blue in the face. Science doesn’t lie. Enjoy your day.

You don't see any exits here.

You are in a small room .
On the floor are hundreds of coins.
It is pitch black. You are likely to be eaten by a grue.


That’s genius. That’s the solution.

I regret reading the spoiler, though. It’s become too automatic because I don’t care about most spoilers. I realise now that I did care about this one.