Love this concept. I still use it to this day when I get a bit confused. Also, the numbers are inverse of themselves. 2x9= 18 9x9=81 Once you get to 45 and 54, you go backwards in reverse from there. I know, confusing, but it can help.
I don't think i ever knew this... i always used the (ex. 9x3= the number below the # being multipled by 9 .. like 2 then the second number is whatever is added to get 9 ... answer 7)... hope that makes sense lol :)