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Teachers' Lounge Discussion: Rounding off-Estimation

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From: Gail 
To: Teacher2Teacher Public Discussion
Date: 2000101215:02:50
Subject: Re: Rounding off-Estimation

Having a visual picture is very important.  I tell my fourth and fifth
grade students a story a primary teacher told me about.  I ask them if
they have ever crossed a street before.  (Of course, I know they
have!!!)  Then I ask them to imagine they are standing with me on the
corner of an intersection, and we step off the curb, but before we get
more than a few steps, a big car comes, and we have to get to safety. 
I ask them where the best place to go would be.  They tell me the curb
I just left is closer, so that is the place to go.

Then I "step off the curb" again, and when I am just a few steps from
the other side, another car comes.  I need to get to safety quickly,
so I ask them where I should go.  They tell me the curb I am closest
to is the one to get to.

I return to the first corner to "step off" one more time, and this
time, wouldn't you know it, just as I reach the middle of the
intersection, a car comes, and I have to get to safety.  I remind the
students I am halfway across the street already, so there is no closer
side.  I ask them if there is any reason why one side would be better
than the other.  They tell me if I were already planning to go to the
other corner, I should just continue, and that is what I would do.

Now that this picture is firmly in their minds, I remind them that
numbers can be arranged on a number line.  I ask them to tell me how
we could count down the number line by tens.  Then I choose a number
and they are to tell me which two tens it falls between.  For example,
if I say 37, they should tell me 30 and 40.  If I say 26, they should
say 20 and 30.

Then we try to figure out what halfway between each pair of
(consecutive)tens would be.  We discover that halfway between 20 and
30 is 25, and halfway between 50 and 60 is 55, etc.

At this point, we can begin to use the story to figure out how the
number will round.  For 42, the two "curbs" are 40 and 50.  2 is just
a few steps off the curb, not halfway or more, so the number rounds to
40.  On the other hand, 29, between 20 and 30, is only about a step
away from the other curb, so it rounds to 30.  For an amount like 65,
which is right between its consecutive tens 60 and 70, it should round
to 70 because it is already halfway there.

Hope this gives you another strategy to try.  :-)
 -Gail, for the Teacher2Teacher service

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