LAST EDITED ON 06-16-12 AT 09:24 PM (EST)Weighing #1: Put 4 on each side: 2 possible outcomes
1) They are balanced => The different coin is in the 4 left out while those 8 used can be seen as standards
A) Weighing #2: Put 2 of those coins originally left out on one side, put one on the other and add one of the “standards”: We now have 2 possibilities;
i- They are balanced => The different coin is the one that had been left out once more. Weighing #3: Put it on one side and put a standard on the other and you will be able to tell me whether the different coin is lighter or heavier.
ii- They are unbalanced => Weighing #3: Take the two unkown coins that had been on the same side and split them into their own side: We have now 4 possibilities:
a- If the two coins had been on the heavy side but are now balanced then the unkown coin that was left out is lighter than all the others.
b- If the two had been on the heavy side and are now unbalanced then the one that is on the heavy side is heavier than all the others
c- If the two coins had been on the light side but are now balanced then the unkown coin that was left out is heavier than all the others.
d- If the two had been on the light side and are now unbalanced then the one that is on the light side is lighter than all the others
2) They aren’t balanced (Here is the tricky part! There are different ways to go about it but the trick is the divide those 8 coins into a 3-3-2 arrangement)
B) Weighing #2: Put 2 from the heavy side and transfer one possibly light coin on one platter. Transfer the 3rd coin from the heavy side and put in on the other platter with one of the 3 remaining from the light side and add a standard (any of the 4 coins that had been left out in the 1st weighing).
This leaves 2 coins from the light side and 1 from the heavy side off the scale.
i- The scale is now balanced => The different coin is among the three left out of weighing #2. Weighing #3: Split the 2 light coins on the scale - If they are balanced then the coin left out is heavier than all the rest. –If they are unbalanced then the one that is on the light side is the different coin and is lighter than all the rest.
ii- The scale is now unbalanced BUT it is heavier on the side where we have 1 heavy, one light and 1 standard. That means that the different coin is either that possibly heavy one or the light one that had been put on the other side. Weighing #3: Weigh the coin that is possibly heavy against a standard: If they are balanced then, the coin left out was lighter than all the rest but if they are unbalanced then the coin that was on the heavy side is indeed heavier than all the rest.
iii- The scale is unbalanced BUT it is heavier on the side where the two heavy coins had been left. We have only 1 weighing left and the different coin is either one of those two heavy coins or the light one that had been left on its original side. Weighing #3: Compare the two heavy coins: If they are balanced then the coin we thought was light is indeed the different one. If they are unbalanced then the one that is on the heavy side is the different one.
After receiving different entries the one that contained the solution although not quite complete belonged to...
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Mike! Mike wins immunity!