How do you teach subtraction with regrouping? - ProTeacher Community



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How do you teach subtraction with regrouping?
Old 02-20-2009, 06:40 PM
 
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This concept is coming up next week, how do you teach it?


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I teach
Old 02-20-2009, 06:52 PM
 
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a poem for this. It goes like this : More on top? No need to stop... More on the floor? Go next door, steal 10 more. Numbers the same? Zeros the game.

I use base 10 blocks to demonstrate what the poem is talking about first, and then I post it all over the room until it is drilled into their minds. They repeat it every time they come across a 2 or 3 digit subratraction problem.
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Old 02-20-2009, 08:12 PM
 
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I do the same poem! Except instead of steal, we knock on the door and ask politely to borrow a 10. It is hilarious to watch kids actually knocking on their desks and saying, "Mr. Seven, may I please borrow a 10?"
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Old 02-21-2009, 04:53 AM
 
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I just taught it this week. I use dimes and pennies only for 2 days before I have them write anything. Some of them get it the first day, most the next. On the second day, I have someone at the board to show the trading.

The poem is cute and I'll use it but I think it's best to really understand what they are dong first before moving on. My advice, take it slowly. It doesn't hurt anyone and it helps the ones who need it. I'm sensitive to that because I was the kid who needed the extra!
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Old 02-21-2009, 05:08 AM
 
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Place value blocks


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Old 02-21-2009, 07:26 AM
 
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digi blocks!! love them.

before you get them into subtraction with regrouping, they should be solid in knowing just how to regroup a number. for example, we spent 2 days on just regrouping numbers. so my students can tell you that 33 = 3 tens and 3 ones OR 2 tens 13 ones. this helps them realize that you are not changing the value of the number, you're just regrouping.

once they are solid in that, i move onto 2 digit - 1 digit subtraction. they use the digi blocks to help them. when they regroup, when they 'take a 10' i make them write +10 next to the ones digit because that's where the 10 is going. then they cross it out and write the new number, after regrouping.

once they are comfortable, we do 2 digit - 2 digit. the trick is getting them to stop and think if it's regular or regrouping. i have always found this concept to be the trickiest to teach!!
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regroup!
Old 02-21-2009, 08:06 AM
 
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Quote:
before you get them into subtraction with regrouping, they should be solid in knowing just how to regroup a number. for example, we spent 2 days on just regrouping numbers. so my students can tell you that 33 = 3 tens and 3 ones OR 2 tens 13 ones. this helps them realize that you are not changing the value of the number, you're just regrouping.
I agree - I spend a lot of time on equality before starting subtraction with regrouping - they need to know WHY they are writing those little numbers up on top. We explain it as "writing the number a different way"

And don't call it borrowing - when you borrow you give it back - regrouping is different than borrowing something!

I make mine ask themselves this question before subtracting each column:

"Is the top number greater than the bottom number?"

Yes - subtract
No - regroup!
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Old 02-21-2009, 08:43 AM
 
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vark - i totally agree that they need to know WHY. often, they get so caught up in just subtracting, and they don't understand why they need to regroup or not. i still have some kids that automatically think if it's a subtraction problem, then they automatically regroup. clearly, they don't get the WHY part of it yet. luckily, we work on this for the rest of the year

i make them ask the same questions when they start to subtract. i also make them use the appropriate vocabulary, when i remember. so they will ask ' do i have enough ones to take away from ____?' if not, they say that they go next door and 'break a ten.'

it's terminology that works better if you are using digi blocks
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3"B"s
Old 02-21-2009, 10:24 AM
 
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Once the kids have had lots of practice with manips (I use place value blocks) I also teach them the various terms we use for trading/borrowing then I teach them this phrase:
If the Bottom
is Bigger
Borrow

We call it the 3Bs
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digi blocks
Old 02-21-2009, 10:54 AM
 
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Just googled digi blocks - very cool!

I use these:

http://www.etacuisenaire.com/catalog...1&prodId=40971

Same idea - they have to physically break the blocks apart or put them together to regroup
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Old 02-21-2009, 02:48 PM
 
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I hope this makes sense. This is what we do...

Ok, I am going to give it a shot... i hope it makes sense.

We use a place value mat and base 10 blocks to teach subtraction. We spend 2 weeks on it. For 2 days we build the problem. We build the top number and then for the number we are taking away, we use index cards. One card for the # in the 10's place and one card for the # in the ones place. We call them subtraction tickets. So if we are doing 41-12, we build 41 and put an index card with a 1 on it in the 10's place and an index card with a 2 on it in the 1's place. We tell the kids to look at the one's. Ask how many ones do we have? How many do we need to take away? Do we have enough? Then we sing:
If there's more on top take a 10,
If there's more on top take a 10,
if there's more on top take a 10 and start again,
if there's more on top take a 10.

It goes to the tune of if your happy and you know it.

So we don't have enough to make a trade. So we take off a 10 stick and put up 10 ones. Then I ask do we have enough to take away 2 ones. Yes we do. Put 2 ones on the subtraction ticket and take it away.... Move the remaining ones down to the answer area. Then do the same things in the 10's place. How many 10's do we have? How many do we need to take away? Do we have enough? put 1 10 on the subtraction ticket and take it away. Move the remaining 10's down to the answer area.

We do that for 2 days. Then on Days 3 and 4 we build and sketch the problem. Sketch like I told you on the addition. For subtraction tickets just write the number and put a box around it. Use a red pencil to cross out a 10 and sketch 10 1's. Then circle what you are taking away and cross it out. move the remaining 1's down to the answer area. It makes it easier to see when you use the red to cross out the 10 and put up 10 ones. Then on day 5 we test them on sketching the problem. For the sketch.. we have a worksheet with the place value mats drawn and the boxes for the subtrahends and answer area already drawn.

The second week, we sketch and use numbers for 2 days and then use numbers only for 2 days and test the last day. When we use numbers, I use a red pencil to cross out the number in my 10's place to change it to one less. And also to change the nu

Be sure to use the questions, how many ones do we have? how many do we need to take away? Do we have enough? Every time you subtract, use these questions. I even write them on the board and leave them for awhile.

I hope this makes sense. If it doesn't, pm and I will try to explain better.
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subtraction
Old 02-22-2009, 08:09 AM
 
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I'm a student teacher and in my 2nd grade we use two different ways. We either say "Bottom Bigger? BREAK!" and If the bottom number is bigger, we break the top and borrow from it's neighbor.

I really like to say "Zap it, shrink it, give the ones a ten" and the kids really enjoy this one. If we're doing 2 digit subtraction, ie) 34-17, we would zap the 3 and shrink it to a 2. Then we would give the 4 a "ten" which would turn into 14. To help them, we put a square box over the ones and tens columns so they can put the new number inside.

Hope that helps!
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I've never taught borrowing
Old 02-22-2009, 08:24 AM
 
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We don't teach borrowing. We use 10 frames and other strategies, since many kids grow up not really understanding place value.

For 42-26, a student might say "I know 26 to 46 is 20, subtract 4 is 16."

OR

For 31-19: 30-20 is 10, add the extra 2 for 12.

Of course, I accept all strategies, including borrowing, but really. No kid can come up with borrowing on his/her own, which just goes to show it is a teacher-centered way of solving a problem.

Just my 2 cents.
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I show it with base ten blocks
Old 02-22-2009, 12:49 PM
 
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and we practice making equal amounts to show the regrouping as being equal.

Then we use base ten blocks and practice just doing that.

Then we just do the algorithm.

Then I go back to the blocks again.

Then back to the algorithm.

Be patient. There will be many that don't get it and get frustrated, but slowly the light comes on for students at different times. It's great when you hear, "oh, now I get it!" Not everyone gets it at once and several may need one-on-one. Some may need to use blocks. Some will be fine with the algorithm. In the beginning I tell them it will be confusing and not everyone will get it at once, but that is what I am there for--to help them learn and that's ok not to get it at once.
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