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Estimating Sums By Rounding

Date: 08/16/98 at 05:53:24
Subject: Estimating Sums By Rounding

Dear Dr. Math,

For our homework, we have to estimate to find the sum or difference by 
rounding. An example is 698 + 249. Do you think my teacher means after 
you get the answer, or to round the problem off before you add it?


Date: 08/18/98 at 15:24:22
From: Doctor Margaret
Subject: Re: Estimating Sums By Rounding

Hi Chase,

Thanks for writing to Dr. Math. One of the reasons we use rounding is 
to try to find something out more quickly than the regular way. For 
example, if you had 20 dollars to spend for school supplies and you 
went to the store and found the following prices:

   Pens     1.98
   Binder   7.19
   Pencils   .99
   Notebook 3.25
   Erasers   .69
   Whiteout 2.79

How would you know whether you could buy everything? Would you stand 
there with a pencil and paper and do the addition? The best way, and 
the quickest, would be to round off right away, like this:

   Pens     2.00
   Binder   7.00
   Pencils  1.00
   Notebook 3.00
   Erasers  1.00
   Whiteout 3.00

Now you can run down the dollar column in your head:

   2 + 7 + 1 + 3 + 1 + 3 = 17

Seventeen dollars - approximately.

So your teacher probably wants you to round off stuff before you 
actually do the addition or subtraction. 

Make sure that you round off to the place that she wants. For example,

   516 rounded to the nearest ten equals 520, but rounded off to the 
   nearest hundred equals 500. This is what you have to be careful

I hope this answers your question. Let us know if we can help with 
anything else.

- Doctor Margaret, The Math Forum
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Associated Topics:
Elementary Place Value

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