---------------------------------------------         
|          |unknown | factor | term | cost  |         
---------------------------------------------         
|shirt     |        |   1.0  | 8.00 |       |         
---------------------------------------------         
|tie       |        |   1.0  |  0   |       |         
---------------------------------------------         
|  total   |        |        |      | 26.00 |         
---------------------------------------------         
unknown = (total cost + shirt term) /
          (shirt factor + tie factor);          
for shirt, tie:                          
   cost$ = unknown$ X factor$ - term$.