> I remember him saying: "Forget all rules about percentages. All you need to know it's like a number multiplied by 100."
I don't remember what rules or tricks I was taught about percentages, but I do remember realising that "%" meant "/ 100" and from there everything else followed.
I went to elementary school in Czechoslovakia, late 80s, and we learned some rules about percentages (which I believe was just a modified formula with the multiplication by 100) as a part of what you call cross-multiplication.
I don't remember what rules or tricks I was taught about percentages, but I do remember realising that "%" meant "/ 100" and from there everything else followed.