Let $n= \overline{a_1a_2a_3}$ ($a_1 \neq a_2$, $a_2 \neq a_3$, $a_3 \neq a_1$).
1/ How many possible values does $n$ have which is divisibled by $7$?
2/ How many possible values does $n$ have which is divisibled by $3$ or $4$?
3/ How many possible values does $n$ have which is divisibled by $3$ but not divisibled by $4$?
4/ How many possible values does $n$ have which is divisibled by $3$ and $4$?