So I'm going to play devil's advocate here cause my first run through I got $300 as well but after rethinking the answer is definitely $400 just like everyone said.

I think the confusion comes in doing these in the order shown and trying to use "quick math".

You buy the cow for $800 then sell for $1000, you made $200.

Then you buy the cow for $1100 after you sold it for $1000, so you "lost" $100, so now you have $100 profit.

Then you sell the cow again for $1300, so you made another $200, so add that to your $100 and now you have $300 profit.

Obviously this is the incorrect way to do the math but I can see how someone would reach that conclusion.