Nice explanation St. Pauli. It it's that I can tell you I find it to be an okay strategy. Labelled math illiterate by my school, I was never asked to do mental math....
This is how I taught myself how to give change when I had a job in high school, wasn't aware it was an actual teachable concept!
For some thinkers I think it's a great strategy for mental math, so maybe teacher isn't just a jerk(although she could be a jerk who is teaching a neat mental math strategy!)
This is a way I could quickly visualize subtraction in my head.
It actually came in handy when I was a bartender in college. I could take drink orders from six different people, take the money all at once and get them all their drinks and correct change. I earned a lot of tips with that particular parlor trick.
