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Q&A #495

Teachers' Lounge Discussion: Rounding numbers to the nearest ten

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From: Gail 
To: Teacher2Teacher Public Discussion
Date: 2000073117:32:05
Subject: Re:  Rounding numbers

I want to second Mary's assertion that place value is an important
part of rounding.  I don't allow my fifth grade students to use the
0-4 and 5-9 "rule" when they round.  Instead, they have to decide if
the number they are working with is more, or less, than half way to
the next consective ten  / hundred  /  thousand   or whatever place
value we are rounding to.

That means they need to be able to determine what the two consecutive
multiples are "surrounding that amount (for example, if the amount is
1538, and we are rounding to the nearest hundred, then the two
consecutive hundreds are 1500 and 1600.  If we are rounding to the
nearest thousand, then the two consecutive thousands are 1000 and
2000.

Once they can identify the surrounding multiples, they need to be able
to locate and name the midpoint on that line (for the above examples,
1550, or 1500), and then they need to be able to identify the location
of the amount they are concerned with on a numberline (for the example
amounts above, 1538 would be before the midpoint for 1550 and 1600,
but after the midpoint for 1000 and 2000).

This practice has paid off.  Because we used the idea of "halfway" so
often, now that we are multiplying decimal amounts, when we have a
problem like .4 X .68, we know we can say that it is about "one half"
times .68, or one half of .68.  The understanding with regard to
rounding has transferred and extended to a new topic.

 -Gail, for the Teacher2Teacher service


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