ANSWER Logic Puzzle: Buying donuts puzzle

Here is the answer to the question that was posted, here. There are a few ways to answer this, you can use the brute force method or you can look for an easier way.  This actually turns out to be a problem space where there is a specific way to solve it, On the Frobeniusââ?¬â?¢ Problem of three numbers.

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?

 

 

Solution
I�m going to show the brut ...