Teacher2Teacher |
Q&A #1234 |
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 Visit us again at http://forum.swarthmore.edu/t2t/
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