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  #1  
Old 11-14-2010, 06:12 AM
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S Dubb S Dubb is offline
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Regrouping Twice While Subtracting

My third graders just cannot grasp this. When we work in small groups or whole class they seem to do fine (unless they're really fooling me!), but they just took a test on this concept and bombed it. I feel like all the work we've done with it was worthless.

When they have to regroup once (two-digit regrouping) they do perfectly fine. It's when we get into the hundreds. That's when it becomes a problem. Does anyone have any neat little tricks or ways to help the kids remember that they might have to regroup in the tens place twice sometimes?
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  #2  
Old 11-14-2010, 07:10 AM
MissJill MissJill is offline
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Don't use any tricks.

I like to show my students how the regrouped numbers add up the original number. They need to know that they aren't losing anything and it is just going somewhere else.

Try using base 10 blocks to show this visually. But whatever you do, do not use any tricks!
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Old 11-14-2010, 07:21 AM
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mopar mopar is offline
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Demonstrate with the base ten blocks---they are magic in math for many students.

Maybe have the students write down a rule for when you need to regroup twice. Let them draw a picture and post it in the classroom as a reminder.
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Old 11-14-2010, 08:00 AM
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czacza czacza is offline
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I have kids do only one regrouping at a time...So regroup the tens to the ones place for subtracting in the ones, subtract. Then move on to the tens and regroup from the hundreds...Kids who do all their regrouping at once before starting any subtraction end up with a messy jumble of numbers up top that can be confusing.
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Old 11-14-2010, 08:32 AM
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S Dubb S Dubb is offline
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I've actually done the base-ten blocks. A few times. Even that isn't clicking with a lot of them.

czacza - I like your idea, and I completely understand your logic too. My only fear is that I've been teaching them one way and I'm nervous that introducing another (one that abandons what I've been doing) would be confusing to them. I suppose at this point, however, I have nothing to lose.
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Old 11-14-2010, 08:39 AM
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ChristyF ChristyF is offline
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I agree with the others Base 10 blocks are the way to go. My kids know where mine are stacked and they are able to go get them any time they need to. First, make them exchange 10 units for 1 rod and 10 rods for one flat. Make them do it over and over. That forces them to see the relationship between the two. Using Base 10 blocks makes the subtraction and regrouping a physical movements that makes sense to them.

* Class management tip - At a class this past summer I got the idea of storing my base 10 blocks in plastic dishes (think ziplock lunch containers). I have 26 sets so that if they need, all 26 kids can have a set. It's SO much easier than storing them all together.

*edit* I agree 1000% about making them do the regrouping one step at a time. That shortcut of doing it all at once doesn't always make sense to kids (it didn't to me). Sometimes shortcuts actually make it harder.
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Last edited by ChristyF; 11-14-2010 at 08:41 AM. Reason: just read CzaCza's post
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  #7  
Old 11-14-2010, 09:21 AM
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amakaye amakaye is offline
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Another tool that helps some of my kids is money--exchange the hundred dollar bill for tens, and then exchange a ten for ones. It reinforces that it has to be a "fair" trade.
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Old 11-14-2010, 10:32 AM
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Peachyness Peachyness is offline
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Quote:
Originally Posted by czacza View Post
I have kids do only one regrouping at a time...So regroup the tens to the ones place for subtracting in the ones, subtract.
This is what I do too. So many times I'll get a group of kids who start to regoup all at once and it just becomes a big ole mess. I then have to force them to slow down and regroup one at a time.

I also do the base ten blocks to show them what this all means.
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