Logic Puzzle: Buying donuts puzzle

I thought it would be fun to try something new here.  So I am going to present a logic puzzle and let people try to answer it.  I will post the solution in the future but I want to give people a chance to try to solve it.

So here is the first puzzle:

A baker sells donuts in boxes of 6, 9, and 20. What is the greatest number of donuts that it is impossible to buy?