Recently I discovered Stan Wagon’s Problem of the Week. This is a delightful mailing list / site and some of the problems are in the vein of puzzles I post here. Recent problem 1125 captured the attention of several Math Factor authors so I thought I’d post the puzzle here as an excuse to introduce you all to that list.
You have eight batteries and know that four are good and four are dead, but don’t know which are which. Your only method of testing them is to insert two into a device that will work if you’ve put in two good batteries and not otherwise. How many such “tests” are required in order to be sure that you’ve located two good batteries?
As of this posting, the answer to this question is not yet on the POTW website, but if you come to this later, the spoiler may be there, so be careful to avoid spoilers if you want to work this through.