đó có mấy chữ số khác nhau:
99999999999999999999999999999999999999999999999999999999990999999999999999999999999999999999999999999999199999999999999999999999999999999999999999999999999999999999999999999999999999992999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999399999999999999999999999999999999999999999999999999999999999999999499999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999995999999999999999999999999999999999999999999999999999999999999999999999999999999
có 5 số là số 1,2,3,4,5
6, tính cả c/s 9.