Three boxes of grapefruit are incorrectly labeled. The labels read "white", "pink", and "pink and white". Visual identification can only be made by cutting the grapefruit and noting the pink or white color. By taking a single grapefruit from one of the boxes and cutting it open, determine how to switch labels to correctly identify the boxes.
This is an old problem, however you have to say all labels are incorrect.
Pick the white box. if the grapefruit is white, it is the white/pink box,
Then, pink is wrong, so it is white, and the old white/pink box is now pink.
Try it with any combo, it works.