You always do the estimating first. That way, you'll know
if your answer makes sense. If not, you know right away that
you made a mistake in adding.
When I look at these numbers, I make the first one about 365 thousand
and the second one about 6 thousand.
I add those in my head, and I estimate that the sum is going to be
about 371 thousand.
Now I pick up my pencil and add the numbers. I get 358,643 .
This is not good. Something went wrong. That's nowhere near my estimate.
I must have made a mistake. I'll have to try again.
This time I put the numbers into my calculator and add them.
I get 371,109 . This is very near my estimate, so I accept it.
I look back at my work on paper, to see if I can spot my mistake there.
Yep ... there it is. On paper, I subtracted the numbers instead of adding.
The estimate ... which I did FIRST ... told me when my answer was
wrong, and when it was right.