Buying shares at a new price whilst already holding stock in that company

So what I want to know is how this affects your position. Say a stock was trading at $20 and I bought 10 shares. Two hours later it’s trading at $30 and I buy 5 shares. Would I get to choose which priced stock I sell? Say it moves to $25, will my portfolio say Im up or down?

The rest of this post is just filler.
Ive tried to find advice on this subject before but never get a straight answer. Anyway, though I’d try this sub but it wouldn’t post due to “low effort”.
Really don’t see the need for such a restriction when a post is tagged advice. If there are any subs which give out this kind of advice please let me know.