>>11628200>>11628226>>11628234>>11628326>>11628334The point isn't being able to prove that you can do it once, the point is to see how many you can do quickly in succession. It's a sport.
I wanted to get better at mental arithmetic and I worked through literally hundreds of the worksheets in OP's picture, timing myself on each page. Once I got comfortable with a column of height 2 I worked on height 3, then 4, then 5, and so on until I was doing hundreds of pages of columns of 10.
What's cool about doing these large quantities is that you gain so much facility with numbers that you can very easily partition them, and when doing multiplication you begin seeing factors as a reflex, like second nature.
We all know partitions and factorization are actually important academic topics so let's not snub our noses at something that leads to greater intuition for it.
