There are nine coins, one slightly heavier than the rest. You have three old-fashioned balances like these that you can use to compare the weights of whichever sets of coins you would like. Exactly one of the balances provides arbitrary readings (neither necessarily right nor necessarily wrong), but you don’t know which balance lies. Find the heavier coin in only four weighings.
This will be the last puzzle from me, because my internship here is drawing to a close. Thanks to all of you for working the puzzles and brightening my day.