Easy once you know how it works.
I see people shitting on common core but it actually makes sense. A large reason why so many people have trouble with math here in the US is because we're basically just told to memorize things. Also you should realize that just because something works on paper doesn't make it's going to work as well in your head. From what I've seen from common core, it teaches you to break down the numbers so that it's easier to figure out without something to write on or a calculator. Common core looks retarded on paper but makes a lot of sense if you actually think about it.