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a) The possible case is: 4 milk chocolates, 4 dark chocolates and 4 white chocolates. The next chocolate gives the necessary variant.
b) The possible case is: 20 dark chocolates and 20 white chocolates. The next 5 chocolates (milk) gives the necessary variant.
THE GENERALIZED PIGEONHOLE PRINCIPLE
If objects are placed into boxes, then there is at least one box containing at least objects.
At least 4 people on campus would drink the exact same amount of water.