As a mathematician I may be the wrong benchmark but back in primary/elementary school (I do not know how you call it in the US) in a completey normal class at t

As a mathematician I may be the wrong benchmark but back in primary/elementary school (I do not know how you call it in the US) in a completey normal class at the age of 12-13 everyone understood the "two column format" and everyone used it. I guess maybe 10-20% of the class coudn't solve equations like this on the fly writing on the board in front of the class. And they were just slower and needed more time than the rest. This was shortly after the fall of the iron curtain in Western Europe. I remember at the age of 12 being once in a math camp with a kid who used to live in the US before and he told us that the american kids in his class could not calculate the sum of 5 and 6 in their head. I thought this was just a mean joke. Until now.