---------------------------------------------         
|          |unknown | factor | term |length |         
|          | length |        |      |       |         
---------------------------------------------         
|piece one |        |   0.5  |  3   |       |         
---------------------------------------------         
|piece two |        |   1.0  |  0   |       |         
---------------------------------------------         
|  total   |        |        |      |   12  |         
---------------------------------------------         
for piece one and piece two:                          
   unknown length$ = (total length + piece one term) /
                     (1 + piece one factor);          
   length$ = unknown lenght$ X factor$ - term$.