Ten bags are full of coins. All the coins look the same, but those in one of the bags weight 1 gram less than those in the other nine bags.
You have a scale like the one shown in the picture, but are permitted to make only one weighing. Any number of bags will fit on either side of the scales.
How do you find out which bag contains the lighter coins?
UPDATE: There are more than ten coins in each bag, the bags can be opened and the pointer on the scale allows you to see the degree to which it is tipping in a certain direction.
As ever, please do NOT post your answers, but feel free to say if you have solved it and how long it took.
Richard Wiseman 
Ooo I know the answer!
Took me about 30 seconds, we don’t discuss answers here though do we?
🙂
No we don’t do answers here until Monday after Richard posts his then we can discuss our answers.
I think you either need a scale that gives numeric results rather than a simple balance, or two weighings rather than one! I look forward to seeing the answer if I’m wrong.
Well I know how to get it to a 50/50 choice which isn’t the answer….hmmm I guess I’ll have to keep giving it some thought and I am sure I’ll find it and think dumb why didn’t I see that right off….lol…
OK now thinking more on how I got to the 50/50 I think I can proceed from there or “continue” with my process to get my finial answer….would or could be sneaky…….. all depends on terminology….
OK after looking up and refreshing my memory on how to use the above scales AND can open the bags I think I have a better answer than the sneaky way to measure although the sneaky way should still work.
Now I know I have the answer!!! yeah…yeah….jump up and down and dance around……yeah ….yeah…. well I knew my coin collecting would one day find a propose and make up for looking nerdy…. I set up an experiment…..did the puzzle….live….. dead wouldn’t have worked as it would have slowed things down greatly…hehe….I have a workable in real life answer!
When you say scales like in the picture, do you also mean with a full set of calibrated weights?
I guess it’s even trickier:
the scale in the picture doesn’t seem to have any numeric scale at the pointer, so it doesn’t seem to indicate the weight difference between the two scales. It seems you only know which side is heavier, and you seem to have no clue how much heavier. And the picture doesn’t show any calibrated weights you can use…
Are we allowed to remove coins from the bags?
I suppose so, but i don’t know why that would help.
If the answer is the one I think it is (you have to think mathematically) taking out coins will actually help quite a bit.
I think I know how to it, but I’m taking to the letter what Richard told us about weighting. If it doesn’t mean what I think it means, I’ll be in trouble.
Very nice puzzle. About 1 minute.
Hey Joao Pedro, I have an answer…..yeah! ….yeah!….I’m happy….maybe not proud of how long and having to work the puzzle in real live action…..but it works! AND I got to play with my coin collection too!
I doubt it’s the right answer, but I got one which could work. It’s way, way too complicated though.
I already solved this puzzle before. But the version is different and the technique is almost the same just a little bit tricky when to find out the final result.
Good I love this puzzle. About a minute.
Is there any calibration? Is each coin, or the entire bag 1 gram lighter? Also curious about the number of coins per bag.
Each coin. It has to be to make the problem solvable.
I think it doesn’t matter
Quantity of information required to identify the lighter coins : 10
Quantity of information provided by ONE weighing using such a scale : 3 : left side is heavier, or right side is heavier, or both sides weigh the same.
–> impossible, or wrongly defined…
I guess we are supposed to be able to tell the weigh difference between the two sides of the scales, using calibrated weighs.
We do not need quantity information. We only need to know that one of the bags is lighter than the others.
I agree with Tom, there are 10 possible answers and the scales can only give 3 different outcomes, impossible, or wrongly defined.
I think the scales tell you how much difference there is in weight between the left and right side. So, there’s a large number of outcomes.
Well I have an instant answer but I await the proper answer to know if I’ve got it or if I am just cheating!
I know a way to cheat the answer out if I had to performed the weighing in real life. 😀
I think that actually is the answer. There simply is no other way to do it.
My solution depends on the definition of “one weighing”.
same here.
I mean would it be alright to add my bags one by one to make the “final” weighting? 😉
I’m totally and absolutely flummoxed by this.
A couple of points which might be relevant.
First – there’s no statement that there are the same number of coins in each bag.
Second – there’s no statement that the bags are the same size.
Without those qualifications I don’t see how you could go for a solution based on weighing whole bags.
“You have a scale like the one shown in the picture.”
A scale out of balance, indicating that the air to the left actually weighs more than the air to the right? Tricky, I’d say.
I was wondering who’d be the first nitpicker to point that out.
Solved in about half a minuets. But followings are the pre-requisites for my solution to be workable:
1. 1. There are ten or more coins in each bag.
2. 2. Bags can be opened.
3. 3. Calibrated weights are provided.
I spent about 1 minute 59 seconds feeling dumbfounded and thinking it just couldn’t be possible, the the answer came to be in a Eureka flash.
Got this same from a colleague at work a while ago. Took me about 30 minutes to get it, I think. This is a clever puzzle. 🙂
30 secs.
It’s a clever one! According to me ,the scale in the picture doesn’t seem to have any numeric scale at the pointer, so it doesn’t seem to indicate the weight difference between the two scales.
Brainteaser – http://www.iqtestexperts.com/brainteasers/
Well I have a solution that seems to make sense and I think would work. Will just have to wait until Monday now to see if I’m right.
Ooh I think I figured it out! And really quick, too. I’m excited! Can’t wait to see if I’m right.
with the update, took about a minute, but i’m sure i’ve seen this one before
Ah, with the additional info I know how this works, probably a minute to be sure. I was completely stumped first thing when I looked at this as I assumed the bags were sealed.
If not for the updates, an impossible problem. I keep telling you, Richard is not that good at wording his puzzles correctly – perhaps that’s for the purpose of the sociological experiment he’s conducting. Just like the train one, yes I did have the right solution, but more data was really necessary to bound the problem, just as in this case.
What’s the sociological experiment? I always thought Richard put these puzzles up (and appends questions like ‘Which photo do you like best?’ to his other posts) just to get us talking, and encouraging us to think a bit more laterally about the posts. And when he puts up a poll, it’s a ‘fun experiment’, maybe to see if there’s potential for a proper experiment later on.
I can’t believe Richard deliberately leaves out vital information in the first instance, and certainly not because he wants us to point out his shortcomings!
I don’t know, I am beginning to wonder if sometimes the puzzles are deliberately ambiguous precisely to provoke discussion, or as in this case an experiment to judge how many people report a solution to an initially impossible puzzle.
1 minutes, 30 seconds to find a suitable approach and 30 seconds to verify.
After some pondering, I have a way of doing it. However, it makes some risky assumptions and feels a bit like cheating. Think like a magician, that’s all I’m saying.
After much discussion at work we have two ways of solving this.
10 seconds, impossible, or misread the question
I found it pretty easily after the update, altough in my opinion quite less than 10 coins would suffice. I am assuming that “the pointer on the scale allows you to see the degree to which it is tipping in a certain direction” means that it shows the difference of weight (in grams) between the two sides of the scale.
I got a solution without using the update, but I suspect it might be wrong if an update was required. It seems to make sense, though
Oh, yeah. It’s definitely wrong.
Without the update, I would have found this quite difficult. With the update it becomes very easy indeed.
Pick up all the bags once with your hand. Hopefully you have a sensitive hand.
Is it each coin that weighs 1g less or the whole bag that weighs 1 gram less?
I arrived to the answer in less than one minute!
I agree to the sensitive hand. Does the constraint include SOLUTION to the answer? If not, I vote that it should be.
I am eagerly awaiting the answer to this one! I know a way to solve it, but it requires:
1) taking coins out of each of the ten bags to put on one side of the scale
2) knowing the gram weight of a good coin
3) a set of standard gram weights to place in the other side of the scale.
The problem as stated seems to indicate that 1) is not allowed, 2) is not known, and 3) is not available.
Yeah. Depending on the definition of ‘one weighing’ being pretty loose.
I realised what I’d like to do in about 10 secs then spent 2 minutes wondering about the wording of the question.
About 2 mintues, but I don’t konw if my answer is right or not
I think I got it, but only if you
a) are allowed to remove coins
b) can exactly determine how many gramms the two sides of the scale differ.
then it is quite easy.
but if not … no clue 🙂
The solution that I’ve arrived at is so simple, and requires none of the additional information, and is therefore probably wrong! That said, I am sure that the puzzle must be solvable without the secondary information.
Give all the bags to my wife. She’ll get through those coins and get the answer in no time.
Without the final bit of info in the update, it is impossible. With it, it’s pretty simple.
Solved this during a particularly boring drive from Bristol to London. At about Swindon actually. Once you work out how to do it you know you’re right.
Yay! Finally got a handle on this one! Took me longer than it should have done to work out the fine detail – I’ll blame that on not quite being awake yet 🙂
So where’s the answer?!I have one – took me about a minute to work out, and doesn’t need any looking into bags, knowledge of the number of coins etc, just an accuarte set of scales + being allowed to remove bags and it still counting as one weighing (hope I can say that now the puzzle is so old!)
could it be.. less coins in one of the bags? he didn’s say that all the bags had the same amount of coins. he said they all had at least 10 coins and they looked like they had the same amount from quick observation.