Some interview questions are notoriously difficult.
All of them are identical except one, which is either heavier or lighter than the rest - it is either hollow while the rest are solid, or solid while the rest are hollow. You have a simple two-armed scale, and are permitted three weighings. First, we'll examine what thought processes an interviewer is looking for, and then we'll discuss one solution. Start with the simplest of observations. The number of balls you weigh against each other must be equal.