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Add 364876 and 6233 and then estimate sum

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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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