43

Originally McDonald's sold chicken McNuggets in packages of 6, 9, or 20.  If you wanted 12 McNuggets, you could buy 2 packages of 6.  If you wanted 35, you could buy a package of 20, a package of 6, and a package of 20.  But if you had a craving for 8, there was no way to buy 8.  There was no way to buy 25.

What is the largest number of McNuggets that would be impossible to buy?  43.