Question 13
Let $\overline{abcd}$ and $\overline{cdab}$ be two 4-digit numbers such that $a,b,c,d$ are four different digits from 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9. If $M$ is the greatest common factor of both $\overline{abcd}$ and $\overline{cdab}$, then what is the largest possible value of $M$?