A Nugget of Wisdom
(also from a puzzle magazine)
“Delightfully Kosher”, a restaurant, sells three different serving sizes of chicken nuggets; 6 pieces, 9 pieces or 20 pieces.
If you want 12 pieces, you can simply order two six-piece servings.
If you want 13 pieces, you are unable to buy exactly what you want.
What is the largest number of pieces that you are unable to order exactly, based on the serving sizes available?