Boxes of Six, Nine and Twenty
A shop sells snacks only in boxes of 6, 9 and 20. An order of $n$ snacks can be filled exactly if $n = 6a + 9b + 20c$ for some nonnegative integers $a, b, c$. What is the largest $n$ that cannot be filled exactly? Give an integer.
Related problems
Source: The "McNugget number" problem, a classic Frobenius-problem example in wide circulation. Statement written for AxiomIQ.