There are many helpful tricks to use in math. I use some of them myself. For instance if someone asks you to multiply 19 X 44 - I just add a zero to the 44 (440), double it (880)then subtract one of the 44s (since there are 19 and not 20 of them). Answer is 836. I can do it in my head much faster than a calculator. Math is full of short cuts like this.
I use that one a lot too. One thing calculator people do not realize is they can't spot it when they've made a small error. We can, we just look at the answer and say "That can't be right". I do not trust myself on a calculator so I wrote a program to read a file of numbers and then add them. That way I can double check the numbers in the file and then let the computer add them. Every month I have to (used to have to) add a huge bunch of numbers. It was a drag.