The COIN Problem

Answer 1

dead simple.
first of all, divide the 12 coins into 3 lot of 4coins, put 2 lot on the scale, if it is balance, then the 2 lots does not have a counterfeit coin, the third lot which is not scale will definately have the counterfeit coin.
ans then you pick whichever 2 coins from the lot that has a counterfeit coin, put it on the scale, if the scale is level , then you can be sure the 2 coins are real, take one away and put another coin on, if it is still balance, the the last one is counterfeit.
or when you use the scale the second time by putting 2 coins on the scale, if it does not balance, you then can be sure that one of the coin is counterfeit, take off whichever coin and replace with another, if it is still imbalance, you then can be sure the coin that you did not take away is fake, if it is balance, then the coin you took away is fake,.