I think the scene between Lance and Keith about a thousand plus ten makes more sense if you look at it as through the ADHD!Lance lens. It seemed a bit off to me that Lance would say “that’s like a thousand plus ten” because he goes to a space academy founded to train and educate astronauts; multiplication seems like a basic necessity. It might have been a little mistake on his part, but it still wouldn’t really account for his negative reaction when Keith corrected him, and thinking about it, if you rewatch this scene with the thought that Lance does or might have ADHD, his being upset makes a little more sense.
I’m willing to bet Lance shows symptoms of blurting, and when some people blurt, they often mix up words or thoughts. For me personally, mix-ups happen when I have multiple ideas at once, and I say them as quickly as possible so that they aren’t forgotten. It’s very easy for me to understand why Lance would have said plus instead of times if he was blurting.
That being said, Lance clearly does not appreciate Keith correcting him and brushes him off entirely. This might be a defense mechanism. I’m 99.99% certain Lance knows 1,000 times 10 is 10,000, and in him trying not to look as if he didn’t know this, he reacted badly to the situation by calling Keith a dropout. And considering how many people took this as indication that Lance isn’t intelligent, I think it speaks a lot about why he did this. Personally, blurting is something I hate doing. It can often work against you because it’s almost like arguing with yourself. If this were a sensitive topic (I’m sure Lance, a person who deals with a lot of self-doubt, would find how others perceive his intelligence, and thus it would be sensitive, coupled with the frustration of blurting), I can see how something seemingly as small as a correction would irritate Lance.
tldr: Lance knows how to multiply, some scenes make a little more sense if you go in with a hc in mind, and Lance blurts and doesn’t like having his blurting pointed out.