If that's a number line, then I don't know what a number line is. I was taught that a number line was a line with numbers from negative infinity to positive infinity, with 0 in the middle. It basically only showed the relative position of numbers, both positive and negative. That's why I said it was simple and basically useless. I took what was then called "new math" back in the early to mid sixties in junior high and high school. The books we used were basically published manuscripts since they were so new; soft cover, typed with a regular typewriter and then reproduced; from Yale University Press.
Your thought process is the same as mine, and the only logical way to do it, really. So if that's the number line way, I guess I do number lines too and didn't know it. I also memorized multiplication tables through at least 12x12 and that's certainly handy. Don't think they do that any more.