basically, there are tricks in your mind that smart people tend to do which help speed up calculation.
Their mistake is that they're trying to teach these tricks in a regimented way in the classroom to kids who still don't know the basics. It's like trying to teach power rule to someone who doesn't know what a derivative even is.
So for example, let's say you have a subtraction
373 - 218
Well, let's do
>400 - 200 = 200
Then do
>200 - 27 + 18 = 200 - 9 = 191
Make sense?
Basically, mental tricks that make the math easier on you.
The thing is, these tricks are often unreliable, and take a long time to get you to the answer. It's better to have a consistent method that gets you the answer every single time without thinking.
Reminder that Common Core was NEVER tested. Most teaching methods and programs go through a rigorous testing phase, where they are tried on sample classes, and then scaled up to a state, and then eventually the national level. But only after tons of papers are written on it's effectiveness, and addressing any tweaks. Common Core meanwhile was implemented at a national level after 0 testing whatsoever. Simply because a billionaire by the name of Bill Gates thought he knew better than everyone, and used his lobbying dollars to force this on the country.
The writer of the common core testing standards literally said in an interview that he was motivated by "racial and social justice"
