Question 21
The International Article Number has 13 digits $ABCDEFGHIJKLM$. Here $M$ is a check digit. Let $S=A+3B+C+3D+E+3F+G+3H+I+3J+K+3L$. If $S$ is a multiple of 10, then $M$ is chosen to be 0. Otherwise it is chosen to be $M=10-t$ where $t$ is the remainder obtained when $S$ is divided by 10. The Code for a certain Article Number is $6901020\square09017$. What is the missing digit?
