What is the correct way to calculate 200 and 45?

Answer 1

There is no correct way nor wrong way. Any method which consistently gives you the correct answer and with which you are comfortable is the correct way for you.

I was aged over 60 when I learned a new way to factorise quadratics: a way which is easier than what I was taught at school all those years ago. My old method was not wrong, but I refer the new one. So for me, that is not the correct method.

Answer 2

That depends, on what you want to calculate. You don't just "calculate 200"; if you know that to be the answer, then there is no need to calculate anything.