An aged merchant of Baghdad was much respected by all who knew him. He had three sons, and it was a rule of his life to treat them equally. Whenever one son received a present, the other two each received a present of equal value. One day this worthy man fell sick and died, bequeathing all of his possessions to his three sons in equal shares. The only difficulty that arose was over the stock of honey. There were exactly 21 barrels. The old man left instructions that each son should not only receive an equal quantity of honey, but each son should receive exactly the same number of barrels, and no honey should be transferred from barrel to barrel on account of the waste involved.

Now, as 7 of these barrels were full of honey, 7 were half full, and 7 were empty, this was quite a puzzle, especially because each brother objected to taking more than 4 barrels of the same description (full, half full, or empty). Develop a linear integer model to solve this puzzle.