Knowing the operations and being able to do them, say on paper, is important. In my teens I worked as a cashier, I was able to calculate change in my head - pr

Knowing the operations and being able to do them, say on paper, is important. In my teens I worked as a cashier, I was able to calculate change in my head - pretty straight forward and no one dies if I get it wrong. If I need to do something more complicated, or where a wrong answer would have a bad outcome, then I want to be sure I'm getting it right by using a machine to help me. I'm talking about knowing the math and recognizing that I'm human.