Apparently this is a difficult math problem that many people get wrong and could explain why people don't make as much money as they potentially could make.
Answer with the math.
He made $20. $10 the first time, $70-$60=$10, and $10 the second time, $90-$80=$10, for a total of $20.
Analysis
Basic economic math, the reason some people are wealthier than others.
The price of the goat has nothing to do with it. This isn't a cash flow problem, it's a profit question.
Assume he started out with $100 in his pocket, at the end of the two transactions he'll have $120 in his pocket.
Help educate everyone you know about how to make a profit buying and selling things. It's a basic skill that everyone needs to know. Not everyone is good at math. When this question was asked in a group most people got the answer wrong. That's scary, help them learn to do basic math correctly, their economic future depends upon it.
If you don't get how this math works you should never trade stocks on the market.
This math is the basic way you make money buying low and selling high.
(Image above is a generic meme from the internet. No authorship identified. Fair use or it's in the public domain).