Back to school, Desperado?
Seems like you're trying to play hockey without skates
Lets do one similar to yours:
10 - 2a = 803a (2a means 2 * a, 803a means 803 * a)
add 2a to both sides:
10 - 2a + 2a = 803a + 2a
10 = 805a ...still with me?
now wrap up:
a = 10/805
a = .01242...
Now go do yours similarly...
This is not your original problem. Your original problem was 8,042*S= 100- S. For the original problem, adding S is correct and you get
You didn't "add S", you added 1000 S!9,042S = 100
Oh, wait, you are using the European notation- 8,042 means "8 and 42 thousandths" which I would write as "8.042". In that case you are correct. I notice that mr fantastic made the same assumption. We wouldn't have been fooled if there hadn't been exactly three decimal places!!S = 100/9,024
S = 11.06
Added in second edit: But then why did you use '.' in your final answer?
The correct answer is either 11,08 (NOT 11,06) if you are using the "," to separate whole number from decimal part or 0.011 if you are using "." with "," separating thousands.