“Why do we subtract one month's amount?
The first year you must start with inventory on hand so you never get the benefit of investing the whole 36,000. So they did you the average method to get a rough return.
“Why do we adjust this for a half year”
I would actually think of that as 33000/2 * .08. We do this because take the average investment is a rough way of finding the return without using the present value factors of each month's cash flow. If we have 33,000 invested the first month and 0 invested at the end dividing the initial investment by two gives us the average amount invested for the whole year.
It is a rough measurement and I prefer the true time value calculation. Since they don't allow us to have a real financial calculator on the exam, we see methods like this.
BEC 84
FAR 80
AUD 85
REG 78