Ah, I remember seeing some chinese summer math workbook shared online where the answers to each problem could be translated into a bit that could be plotted in a giant grid on the last page.

When completed, it would show a QR code, that upon scanning would mark completion of the workbook and show you the answer key. The implicit idea being you didn't have to be perfect, just good enough so the error-correction algorithm of QR codes was enough to pick up the final image. (Probably extra credit if you could figure out how to do as few problems as needed and then use EC to still figure out the secret link)