How does turning subtraction into addition make people understand it?
You are subtracting, bigger to smaller. Simple.
Common Core is just a case of people with too much time on their hands trying to fix something that wasn't broken.
We're lagging behind the rest of the western world when it comes to education, and you say "if it ain't broke..."
The point is ideas like using addition to solve a subtraction problem, or vice versa allow students to see how all of these concepts are interrelated. Math is more than just memorizing an individual set of rules for each type of operation.
It's the difference between memorizing rules, and understanding.
If we're lagging behind a country in math, say like Japan, it's not because we needed common core. We were lagging because Japan and many other countries put more emphasis on Home work for their kids and it is actually made a priority. Here in the US, you can see most of the kids after school running around on the street or they are on their phones. there is no discipline. Of course teachers could never never tell parents their kids needed more discipline at home and spend hours on their work, so instead they have to say there was something wrong with the system that was good enough to put people in space and give us computers.
And I disagree that the old system was just memorization. Before kids do problems like 25 + 15 they learned how to add groups of things or take away from groups in a way that is more visual. Again, it could come down to having either a good teacher or a crappy one who cannot explain things.