We have three boxes of which we cannot see its contents although we know that one contains black marbles, another contains white marbles and the last one contains black and white marbles. Each box has a label that indicates the content: White, Black or mix, but all are misplaced so that the label of the box does not match its content in any case.
How many boxes will we have to open in order to correctly label all three?
Simply open a single box, which is marked with the label mixture.
This will contain marbles of a single color so we can deduce that the box with the label of that color will contain the marbles of the other color and the remaining box will be the one that contains the mixture.