IPAddr

Whenever you transpose two digits in a number the error is ALWAYS divisible by 9. In fact the error is always either 09, 18, 27, 36, 45, 54, 63, 72, 81 or 90. If you add up the two digits of all of these numbers you will see that (in addition to the number being a multiple of nine) the digits also add up to 9.

In fact the errors can be any of these numbers multiplied by any fact of 10 as well (eg 09,90,900,9000 or 18, 180, 1800 etc)

If you look up the error amount in this table it will tell you what are the possible combinations of numbers which can be transposed to make up that error - eg if the error is 36 then the numbers could be 15 and 51 or 48 and 84 etc)

Difference=09
Amt1Amt2
0110
1001
1221
2112
2332
3223
3443
4334
4554
5445
5665
6556
6776
7667
7887
8778
8998
9889
Difference=18
Amt1Amt2
0220
1331
2002
2442
3113
3553
4224
4664
5335
5775
6446
6886
7557
7997
8686
9779
Difference=27
Amt1Amt2
0330
1441
2552
3003
3663
4114
4774
5225
5885
6336
6996
7447
8558
9669
Difference=36
Amt1Amt2
0440
1551
2662
3773
4004
4884
5115
5995
6226
7337
8448
9559

Difference=45
Amt1Amt2
0550
1661
2772
3883
4994
5005
6116
7227
8338
9449
Difference=54
Amt1Amt2
0660
1771
2882
3993
6006
7117
8228
9339
Difference=63
Amt1Amt2
0770
1881
2992
7007
8118
9229
Difference=72
Amt1Amt2
0880
1991
8008
9119
Difference=81
Amt1Amt2
0990
9009