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system
if you buy something and sell it later on the same day, how do you calculate 'investment'?
user
nothing wrong with the other answers, but here's a "trick" to hopefully make it totally transparent.   imagine that you're not the one implementing this business plan, but someone else is.  let's call this other person your asset manager.  so on the first day, you give your asset manager $9.  he takes this and generates $1 profit from it, recovering the $9 which he then reinvests to generate $1 profit every day.  from your perspective, you just gave him $9.  at the end of the year, he gives you $365 in addition to your original investment of $9 (in real life he'd take the fees of course, or perhaps he's been lending out the money he's been accumulating and taking the interest from that as pay for his services).  so your return on investment is 365 / 9 * 100 % > 4000 %, as claimed by your source.
assistant