16 is the total, and if you divide that up by 4, you get 4 (you divide by 4 because it is in fourths). Because Byron carried 3/4's, you multiply four by three to get twelve. There is only one more fourth to complete the whole to get to 16, so Jim is carrying four bags.
To check this, we can take Byron's 12 bags (3/4), and subtract that from 16 (4/4):
