A man is the owner of a winery who recently passed away.

In his will, he left 21 barrels (seven of which are filled

with wine, seven of which are half full, and seven of which

are empty) to his three sons. However, the wine and barrels

must be split so 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 and no wine is allowed to be thrown away.

How can the barrels and wine be evenly divided?