Subtraction is Hard
Subtraction as a mathematical operation isn't all that difficult – most of us figure out how to do it in elementary school. But I learned recently from a Happiness Lab podcast that adults have a hard time solving problems that require taking something away rather than adding something.
Not surprisingly, kids find this a lot easier.
The Happiness Lab with Dr. Laurie Santos: The Happiness of Subtraction
The podcast recommended a book by Leidy Klotz on this subject that I've put on my reading list.