IN RECENT YEARS a number of clever coin-weighing or ball-weighing problems
have aroused widespread interest. Here is a new and charmingly simple
variation. You have 10 stacks of coins, each consisting of 10 half-dollars. One
entire stack is counterfeit, but you do not know which one. You do know the
weight of a genuine half-dollar and you are also told that each counterfeit coin
weighs one gram more than it should. You may weigh the coins on a pointer
scale. What is the smallest number of weighings necessary to determine which
stack is counterfeit?