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      S = C + xC 
Add - C to both sides:
  S - C = C + xC - C
The C and the - C cancel out on the right
  S - C = xC
Divide both sides by C
 S - C     xC
------- = ----
   C        C
The C's cancel on the right:
 S - C     
------- = x
   C        
That's the answer.
Edwin