Code:
(*m m^(-1) ≡ SoDu [mod n]*)
PhanTuKhaNghich[m_, SoDu_, n_] := Module[{temp, i, j},
temp = {};
i = 1;
While[i <= n,
j = SoDu + i*n;
If[Mod[j, m] == 0, temp = Append[temp, Mod[j/m, n]]];
i++;
];
Return[Union[temp]];
];