When common core math was introduced across the US, I wanted to know what all the hubbub was about, so I looked into it. Funny that, despite so many parents decrying it, a few instruction pages ended up giving me (as an adult) the number sense that hadn't fully developed from my time in school. I use constructs from it all the time now and mental math has never been easier.

Calculators are great tools, but being able to do quick math in your head before everyone else can finish punching the numbers in makes people wonder if you have super powers.