Add 364876 and 6233 and then estimate sum

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.

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