I wonder if you guys know the answer for this problem:

Problem:
There are two kinds of coins, genuine and counterfeit. A genuine coin weighs X grams and a counterfeit coin weighs X+delta grams, where X is a positive integer and delta is a non-zero real number strictly between -5 and +5. You are presented with 13 piles of 4 coins each. All of the coins are genuine, except for one pile, in which all 4 coins are counterfeit. You are given a precise scale (say, a digital scale capable of displaying any real number). You are to determine three things: X, delta, and which pile contains the counterfeit coins. But you're only allowed to use the scale twice!


Source:
http://research.microsoft.com/...hing%20piles%20of%20coins

I've seen and solved this with 3 weighings. But wit 2? I don't see it.

All 13 piles on the scale. 1st
12 piles on the scale . 2nd

Case 1 :

The removed pile is with authentic coins. Divide pile weight by 4 => X
(Total weight of all the piles (minus) X x 4 x 13)/4 = +/- delta
Which pile contains counterfeit - well i don`t see that coming except if you do multiple 12 piles scale.

Case 2 :
The removed pile is with counterfeit coins. You will figure that you picked the counterfeit pile since the math does not add up when you do the total weight.
The X would be figured by dividing the 2nd scale with 48. The delta will be figured dividing the weight of the "counterfeit pile" by 4 minus X.

In this case you have all the answer.

Can`t figure how would you determine the counterfeit pile in Case 1.

Not really sure if gives you the answer you were seeking. This is just me taking a imaginary snap-shoot if i were to be working in a bank and had to do that :P

i think i'd start by breaking the arm of whoever says i can only use the scale twice, and go from there.

Google can help you: http://www.numericana.com/answer/weighing.htm

no wonder I cant net, what foreign language yall speaking there. It all looks like gobblydook to me.

nerdssssssss.jpg

my guess is that it has something to do with the integer part and that you have to mix piles, but I couldnt come up with an answer and now I need to play football

I start by finding 11 friends...

exactly.

trumper provided THE answer ( link ).

trumper's link explains with 3 weights no with 2.

as usual, microsoft rips off an age old riddle and claims it as their own:P
I remember hearing this riddle as a kid involving gold and fools gold.

:P

Agrees on this method, ill do it this way and start from there

wonder why Columbo was only allowed to weigh it once on a penny scale... ahh. inflation i guess.

drop the coins onto a hard surface (eg glass), legit coins make a totally different noise from fakes from the accoustic attenuation (sound conductivity) of the different metals - also works inside a plastic money bag

drop 1 by 1? that takes "bit" time.

lol. I agree that it can be solved with 3 and not with 2. lol
So the problem must be some typo! LOL

i think if you can't tell by looking at them then just take them to the bank and get cash!

bonus