[Catphish]

It’s grammar school math. The author stated incorrectly that
“13 – 3 = 10 – 1 = 9” and I corrected him.

[Baynative]

I geuss this shows my lack of understanding. “13 – 3 = 10 – 1 = 9” looks right to me.

I cant see how 13-3-1= 10 unless you work backwards and deduct 1 from 3 first then reason that there is now 2 left to deduct from 13. But, then you'd be left with 11. However going the other way it looks like 9 to me.

I still relate to the old vertical columns that have worked since someone figured out numbers.