The Twelve Balls
Twelve visually identical balls include exactly one counterfeit that is either heavier or lighter than the rest, but you don't know which. Using only a balance scale, what is the minimum number of weighings that always suffices to identify the counterfeit and determine whether it is heavy or light?
Related problems & prerequisites
Worth solving first
- Three Switches, One Bulb 6.0/10
- Two Eggs, 100 Floors 6.0/10
- The Mislabeled Jars 6.0/10
Source: Classic twelve-coin balance-weighing puzzle, in wide circulation since the 1940s. Statement written for AxiomIQ.