A frustrated parent responds on a Common Core assignment. Looks like it's back to the old New Math.

[mike_c]

92. an alternative perspective....

I think many folks are missing the point here. Yes, memorizing addition tables and performing simple subtraction yields an answer more simply, but finding the correct answer is only one possible objective. Understanding the computational process that gives context to subtraction is another, and this exercise seems better designed to do the latter, i.e. "use simple units that are multiples of the number system base to converge upon a general result." The process is universal, and can be applied whether one knows addition tables or not.

If the goal is simply to teach kids how to subtract quickly, then the rote method is probably better, IMO-- at least, it worked for me back in the ancient days of my public school education, LOL. But if the objective is to foster understanding of WHY the rote method works, this approach seems better. The powers of ten approach will work whether one knows the tables or not, at least in base 10 arithmetic (and it works in other bases too, with modification to reflect the base change).