-----------------------------
|  old |unknown|weight|digit| 
-----------------------------
| units|       |     1|     |
-----------------------------
|  tens|       |    10|     |
-----------------------------
|number|       |      |     |
-----------------------------
-----------------------------
| new  |unknown|weight|digit| 
-----------------------------
| units|       |    10|     |
-----------------------------
|  tens|       |     1|     |
-----------------------------
|number|       |      |     |
-----------------------------
old's tens's unknown =
     (new's units's weight X
      old's number unknown + 18 - old's number's unknown) /
     (old's tens's digit - 1 + new's tens's weight);
old's units's unknown =
      old's number's unknown - old's tens's unknown;