HARD: 12 Balls and a Scale
By dcb on Feb 22, 2005
I found it! This logic puzzle appears simple. But when I was challenged with this about 12 years ago (by my good friend Scott Bardsley - now a chip designer at Analog Devices) it took me two days to figure it out! There are no tricks. Yes, there is a solution... I do really mean three (3) moves. And the solution must work in the general case (every time). Don't start this unless you have some time (eg: flying across the pond).
You are given 12 billiard balls that all appear identical in color, size, weight, texture, composition, etc. But one of the balls is either slightly lighter or slightly heavier than the other 11. You have a simple balance scale. Describe the process that will allow you to determine which ball is different, and if it is heavier or lighter. Oh... you can only use the scale THREE times.
In case you give up, here is the solution: