Christmas Present!! (Brainteaser)

Let's start off by defining some rules.
1) please no one post answers directly on this post, rather send all answers to me via a message.  Comments will be deleted and the person will be disqualified from all future contests.
2) The first 5 people to respond correctly to this post will receive a free 1 hour tutoring session via the online platform in any subject that I am approved in.  I will respond back to your message explaining the correct answer, how to get that answer, whether you were correct, and, of course, the details in setting up your free session with me
3) If you are trying, but stumped...message me for a hint
4) Have fun!!
Now for the brainteaser!
Chicken McNuggets can be purchased in quantities of 6, 9, and 20 pieces.  What is the largest amount of McNuggets that can NOT be purchased, using these quantites?
Happy Holidays everybody!  I look forward to hearing answers from all of you!!
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