R.E.N. Spells Ren
Veteran
Once again, at the very least, he would need $70 in order to be able to make both transactions, so that's the simplest place to start when attempting to figure this problem out
You could assume he started with $70, and you'll still get a $20 profit (cuz it's the same with ANY starting amount, as I said before)
He starts with $70 cash. He buys a goat leaving him with $10 cash and a goat. He sells that goat for $70, leaving him with $70+$10=$80 cash. He then takes that $80 and buys the goat again leaving him with $0 and a goat. Then he sells that goat for $90, leaving him with just $90 cash.
He started with $70 cash, now he has $90, so he made $20. That's his profit.
It's really that simple, and like I said, it's $20 regardless of what he started with (provided he can borrow money for free, if needed).