Mmm
There are two trades in the first riddle.
In the first trade you buy for $800 and sell for $1000. That’s $200 profit.
In the second trade you buy for $1100 and sell for $1300? That’s also $200 profit.
That makes $400 in total. The fact that it’s the same item that is bought and sold twice makes no difference. That’s a red herring. (I hope I got this right because it’s trades like this that I’ve made a living for the past 20 years nearly.)
The second one I didn’t try to get to terms with, seems more tricky.