There are 21 barrels of wine with a farmer. 7 are full, 7 are half full and 7 are empty. He wishes to divide them among his three sons such that each son has the same number of full barrels, the same number of half full barrels, and the same number of empty barrels. Note that there are no measuring devices handy. How can the barrels and wine be evenly divided?
Use 4 half filled barrels to fill 2 empty barrels. We are then left with 9 fully filled barrels, 3 half filled barrels and 9 empty barrels.
Each son gets 3 full barrels, 1 half-full barrel, and 3 empty barrels.