You have 5 bags of gold coins. The bags are numbered 1 through 5.
Each bag contains 100 coins. Each coin supposedly weighs one Troy ounce.
But, one of the bags contains fake coins made of gold-plated lead.
Each fake coin weighs only 0.9 Troy ounces.
I have a very accurate digital scale which I will let you use for only one weighing.
In just one weighing, how will you determine which bag contains the fake coins?
You can do this, perhaps, by weighing together a different number of coins from each bag, and seeing what the total discrepancy is between the recorded weight and the weight you’d have recorded if they were all gold?
Say 1 coin from bag 1, 2 coins from bag 2, 3 from bag 3, etc etc?
Now you’re weighing 15 coins together, and can work out from the difference between the actual weight and your theoretical 15 troy ounces how many coins are lighter and therefore which bag they came from, since you took a different number from each bag.
It doesn’t need to be 1 from bag 1, 2 from bag 2, etc, that’s just one example, but you need to be able to work out from the weight discrepancy the number of weighed coins affected, so you need a different number from each bag?
Well I have no idea, but it does look like hanukkah gelt. 
The Ever I will Be Here All Week And Try The Prime Rib VIPER
Excellent.
Did you work this out yourself, or look up the solution online?
I worked it out.
It’s only logical, really: there’s nowhere else to go, for a possible solution: if you’re only allowed one weighing, all you can ever learn is going to be from a comparison between the “observed result” and a theoretical “expected result”, so you have to be able to draw a conclusion from the difference between the two, and that means that the magnitude of that difference has to give you information about which bag’s coins accounted for the discrepancy, so they’ll have to be five different possible discrepancies if there are five bags?
so we take one coin from each 5 bag and put it on scale all at once and we every time take one off the scale and take difference and repeat this to all get off and find the one with different weight , something like that ?
That’s multiple weighings, surely?
so really i dont understand it
[quote=“TacitaTrader, post:5, topic:115501, full:true”]
I worked it out.[/quote]
Kudos!
Correct.
Also correct.
this doesnt make sense to me if its true answer
and i think its more like multiple weighting
how about not scaling and use our hands and pick the lighter bag its 100 coins 0.1 each coin difference so its guessable 0.1X100=10 ounce or 310 gram its says troy ounce so One troy ounce is equal to 31.1034768 grams. , if the guy is smart enough making that question he can guess which one is lighter without scale and just hands and put the fake coins bag on scale 
the bring in on scale its where they trap you to use the scale
[quote=“m.k.a, post:10, topic:115501”]
this doesnt make sense to me if its true answer and i think its more like multiple weighting [/quote]
It does work like that and it is only one weighing.
There is only one bag amongst the 5 bags that contains fake coins.
By taking 1 coin from bag no. 1, and 2 from bag no. 2, and 3 from bag no.3 and so on, then you will then have a total of 15 coins in your hand from all five bags. (nothing weighed yet).
If ALL these 15 coins were real then the total weight should be 15 troy ounces. But at least one of them is fake (if bag no.1) and at most 5 of them are fake (if bag no.5).
Now you place all the coins on the scales in one go. The only possible outcomes are: 14.9, 14.8, 14.7, 14.6 or 14.5 troy ounces, depending on how many fake coins, 1-5, are amongst those 15. If the actual weight is 14.9 then there is only one fake coin, i.e. bag 1. if the actual weight is 14.8 then there are 2 fake coins, i.e. from bag 2…and so on.
Clever! (congrats to TT!)
after some internet search yeah easy
Haha! Nowadays that certainly is often the case! Not necessarily a bad thing, sometimes, anyway!
I’d just grab the bags n run.
You’ll find the fake bag on the sidewalk later.
bahaha a legend 
There speaks a true trader!!!
Thats genius.
I spent 30 mins thinking about it and came to the conclusion that it was impossible.
Good job
That is EASY!!!
If you take One coin from Bag 1, and 2 Coin from Bag2, and 3 coin from Bag3, and 4Coin from Bag 4, and 5Coin from Bag 5. and Weigh the Lot(1+2+3+4+5=15Coins).The weight would tell you Exactly, were (Which Bag) the fakes are from, Because it is going to be divisible by .9 ! Get it, Easy
I give you an Example. If the Weight shows 1470 (100+200+270+400+500) = Bag 3 False, (270/90)
