Twelve Coins

We received a nice reply to our puzzle about 12 coins of equal denomination but one was a counterfeit. With a balance, determine which is the counterfeit coin, bearing in mind that this coin can be lighter or heavier than the others.

George Chambers from the Toronto Timex Sinclair Club took the time to develop a program to do just that, all the user has to do is to put in the coins for the program to test. As the intent is to do this with the least number of weighings, group weighings are required. George claims to obtain an answer is 4 tests. I tried several times and obtain the correct answer in 5 weighings. I stumbled for a while on the fact that the computer only tells which side of the balance is heavier, not because one side had the heavier coin. In other words, one side could have the coins on the balance all equal and the other side could have a heavy coin, making it the heavy side, or it could have a light coin, making the side with equal weight coins heavy–tricky!