Twelve Balls + 3
I received my set of twelve balls from the pool hall, so that I could test my solution to the twelve ball problem. But when I opened the box I saw to my horror that the manufacturer had included three extra balls! Now I had 15 identical looking balls, only one of which is either heavier, or lighter than the others. Was it even possible to solve the puzzle with three weighings on an equal arm balance? And if so, how to do it?