Hi, Chuck W.;
Not really "plug-in," as I apparently don't remember my Algebra that well,
but Excel's Goal Seek gave me these results:

Person Service Sales Product Sales Total Sales Ratio
100 $20,000.00 $13,307.80 $33,307.80 40%
200 $30,000.00 $19,990.53 $49,990.53 40%
300 $40,000.00 $26,615.61 $66,615.61 40%
400 $50,000.00 $33,330.27 $83,330.27 40%

Total Sales was Service Sales + Product Sales, for Ratio I used Product
Sales / Total Sales, and for Goal Seek, I asked Excel to change the value of
a "Ratio" cell to .4 by changing the value of the corresponding "Product
Sales" cell. I repeated the Goal Seek on the other three "Ratio" cells. Goal
Seek is on the Tools menu.
Hope this helps.
Ian.