Your 1st line:
“Well, after first month it has to be left 9000*20%=1800 units”
Why do you take 20% from 9000 which is the figure for month 2 and you are not taking 20% from month 1 (8000 units)?
I can kind of see your logic but when I’ve read the questions I assumed that e.g. we need to take 20% from month 1 and add it to month 2 if it makes sense…
