We have a 14-year-old, so have seen similar stuff. I think a lot of what they're trying to do with Common Core is explicitly teach kids all the shortcuts that math-literate people use instinctively, without formal instruction. The problem is, those shortcuts shouldn't really be taught, or at least not as the main technique. You should teach the basic math facts and methods as simply as possible, then let students figure out the fancy ways themselves. Otherwise you just create confusion.
For example, if I asked you, "what is 98 + 87?" you would probably intuitively subtract 2 from 87 to get 85, then add 100 to get 185. Schools are now trying to teach this as a rule called âoeMaking a Hundredâ, âoeFriendly Numbersâ, or âoeUsing Compensationâ, which is unnecessarily complex. They should just focus on the classic "add in columns and carry over" (which is easier to teach and learn), and let people figure out the weird tricks on their own. Then everyone gets some basic proficiency and the people who use a lot of math would figure out the shortcuts on their own. But when you lead with the shortcuts, it just turns into a muddle.