@laurenshof now starts the real challenge: what was the original source of this … shall we be kind and call it “calculation”?

My bets are maybe a mangled combination of this doc (includes “brothers”, “sisters” and calculations written out including numbers): https://files.eric.ed.gov/fulltext/ED036430.pdf

Maybe in combination with something like: https://slideplayer.com/slide/9871551/

But if someone finds a word-for-word copied source, that would be even more awesome.