Without paper, I can’t, provided the numbers are large enough. It wasn’t expected of us to take Math in 11th or 12th grade in my high school. Outrageous, I know, although that was 25+ years ago.
What I described in post #46 was second to seventh grade arithmetic.
Without a solid understanding of these principles algebra would be impossible. Fractions, specifically, are a large part of algebra, although letters, instead of numerals, are used to represent quantities.
Honestly, because you have missed out on the basics, whole swaths of the economy are closed to you. Many, many opportunities for engaging, interesting, highly creative, and often, highly paying work are solidly shut against you. With a little effort, some time at the community college, and some payment of tuition this could be completely overcome.
And...You really might surprised yourself how smart you really are in mathematics.