## Monday, August 27, 2012

### How to Subtract the Common Core Way

It seems we have started back to school a good bit earlier than many of the rest of you, so I thought I might share our initial experiences teaching our first math unit after adopting the Common Core.  It may make it a bit easier to see the anchor charts we created.  I hope this proves helpful.

According to the Common Core, there is an emphasis on building number sense with third graders.  Our first unit has included Addition and Subtraction.  To build number sense, we have taught several strategies for addition and subtraction: using a hundreds chart, an open number line, place value blocks, and expanded form.

Subtraction was the most challenging, especially for informing parents who want to help their children.  Not having text books written for common core makes homework a challenge.  In order to meed this need, we used a Math journal.  In that journal, we had students record an example homework problem that we did together in class.  Then we pasted small homework handouts (half sheets) underneath.  In a perfect world, this would have been just fine....but it's not a perfect world.  The problem we have run into is with those sweeties, that have trouble copying from the board or chart (for the homework example).  This copying thing is still a challenge for many 3rd graders.

I intend on letting my parents know they can check out this blog too, in order to see these anchor charts as well.

This anchor chart shows how to subtract using an open number line.  The hardest part for the students is using the skill of counting backwards, especially when going back over a ten-mark (ie 212, 201, 192).

(*I just realized I left off the letter "e" in the word "increase" below.  I'll fix it soon, upload a new photo and delete this comment, but I wanted to address it until I get the chance to make the change.)

Here is my anchor chart for using place value blocks to subtract when there is no need to regroup. We started with real place value blocks.  Then we moved to the picture model.  After a bit of modeling, my kiddos really caught on to this strategy well. When subtracting without regrouping, this method tended to be especially easy for them.

However, when regrouping was needed, it got more complicated, but after a couple of days, my students found that they still preferred this strategy.  Here is the anchor chart to teach subtracting with place value blocks when regrouping was needed.

(technical difficulty with photo - will be corrected soon)

If you have questions, I will be glad to try and answer them.

1. subtraction is one of the four basic binary operations; it is the inverse of addition, meaning that if we start with any number and add any number and then subtract the same number we added, we return to the number we started with.Definition of Subtraction

1. You are correct. Before 3rd grade, this is generally taught with "fact families" in K through 2. In 2nd and 3rd we introduce the "properties of" each operation, and most seem to easily grasp this inverse relationship.For my kiddos, the operation itself with larger numbers (with the need for regrouping) is the tricky part.

2. What?!?! Your 3rd graders struggle with copying information off the board too?!?! I thought it was just mine!!! ;) Wonderful anchor charts. Thanks for sharing. Amy

1. No, it is definately not just yours! :) I have a student teacher this year, and she was very surprised at this!

3. Your anchor charts look great! My 3rd graders have the same problem too! I think part of it is that it takes too much time to copy things (that's what they think). They are so used to everything being instant with all the gadgets and computers they are used to dealing with.

On another note, I am a fellow 3rd grade teacher and I just found your blog through the Teachers NB forum and I'm now following you! I would love for you to visit my blog when you get a chance!
Patti :)
A Series of 3rd Grade Events

4. We've been using number lines for subtraction for a couple of years and if you can get the idea across that subtraction just means finding the difference between to numbers (how much space in between) it really is easier for kids to jump forward instead of trying to get them to count backwards. Kind of like counting change back at a register. For example is something costs \$2.99 and you give them a \$5 make a number line start at \$2.99 and 1 cent to make \$3 then \$2 more dollars to get to \$5. Accuracy will go way up if you can get kids going forward instead of jumping backwards.

5. What Method is this called?

32-12 (is the problem)

12+3=15
15+5=20
20+10=30
30+2=32
add the center column up and you come up with the answer. Can somebody break this down for me?

1. Hi Fred. Lewis Murray left you a good response below. You are basically just counting up from 12 to 32, and aiming at what many call "friendly numbers" or "benchmark numbers" as you count. We use these friendly numbers because they are easier to use in mental math, when no paper is used.

6. This is basically counting from 12 to 32 in steps, using 5s and 10s when possible. The first step gets us to a numeral ending in 5, the second to one ending in 0, the next adds even 10s to get to the closest numeral ending in 0, the last step finishes up. The left column is the tally counting up. So 12 + 3 + 5 + 10 + 2 = 32. It mimics the way some people do math in their heads. If the question were 92 - 12, it might be 12 + 3 = 15, + 5 = 20, + 70 = 90, + 2 = 92, so the answer would be 3 + 5 + 70 + 2, or 80. Hope that helps.

1. The way I would do that problem in my head is 2-2 = 0 and 9-1 = 8. Therefore the answer is 80. So much more simple.

7. Sorry but no I would do 92-12 as 2-2 and 90-10 no ones and 8 Tens equaks 80. Asking kids to remember 4 addition probs and add them up. Thats just nuts. And to do it mentallly. I get how to do it think it's just so unnecessary.

8. These unnecessary steps will make the gifted kids hate math. For the kids at the lower end, more steps means more opportunity for careless mistakes or a chance to forget steps. And you would learn a method you would never use in real life why???

9. I'm glad that you explained the method. My child is in second grade is autistic and knows all his times tables and can do math in his head. I needed to learn the pattern so that we can work on this. I don't think it will make a gifted kid hate math. I respect the teacher that posted the process clearly. Not the teacher's fault that public schools are being dismantled.

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11. I'm so frustrated that 3 digit subtraction seems to have really killed my daughters confidence.....ex. 402-176 734-182 how can I help her? please

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