McNuggets problem

From Wikipedia:

The McNuggets version of the coin problem was introduced by Henri Picciotto,
who included it in his algebra textbook co-authored with Anita Wah. Picciotto
thought of the application in the 1980s while dining with his son at
McDonald's, working the problem out on a napkin. A McNugget number is
the total number of McDonald's Chicken McNuggets in any number of boxes.
In the United Kingdom, the original boxes (prior to the introduction of
the Happy Meal-sized nugget boxes) were of 6, 9, and 20 nuggets.

Task

Calculate (from 0 up to a limit of 100) the largest non-McNuggets number (a number n which cannot be expressed with 6x + 9y + 20z = n where x, y and z are natural numbers).

C

<lang c>#include <stdio.h>

int main() {

   int max = 0, i = 0, sixes, nines, twenties;

loopstart: while (i < 100) {

       for (sixes = 0; sixes*6 < i; sixes++) {
           if (sixes*6 == i) {
               i++;
               goto loopstart;
           }

           for (nines = 0; nines*9 < i; nines++) {
               if (sixes*6 + nines*9 == i) {
                   i++;
                   goto loopstart;
               }

               for (twenties = 0; twenties*20 < i; twenties++) {
                   if (sixes*6 + nines*9 + twenties*20 == i) {
                       i++;
                       goto loopstart;
                   }
               }
           }
       }
       max = i;
       i++;
   }

   printf("Maximum non-McNuggets number is %d\n", max);

   return 0;

}</lang>