DavidHG — Cute trick…but most of those cute tricks are too confusing for the average person to remember.
A real grasp of numbers works better.
How can you multiply any number by 11 so it makes sense?
11 of anything is ten of them plus another one.
Ergo…11 times 57 is TEN 57’s plus another 57…
570 plus another 57….
And how do you add that in your head?
Several different ways…. easiest to teach has been 570 needs another 30 to make 600… take it from the 57….Now you have 600 and you still have 27 left over from your 57…627.
By the same token NINE 57’s Is 570 MINUS 57.
Might sound complicated but once I get people started with real objects…meaning I’d have them doing that with nickels and quarters and cookies and such before abstract numbers…it starts to ….um… add up.