Question

Add 364876 and 6233 and then estimate sum

Answer 1

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.