Tuesday, January 29, 2013

Oh no, Subtracting with ungrouping!?!

Ah yes, Subtraction with ungrouping.  One of the 3 toughest topics to teach second graders (at least I think).  This goes along with counting money and telling time.  Why oh Why are these so hard, I don't know.. all I know is that I need to take some extra time and give a little extra love to these 3 Math babies..

My district uses Math Expressions for our Math program.  I.Dislike.It.Alot!  The lessons are jam packed, and I just don't get the sequence they have most of their units in.  So I usually just got through the unit, look at the strategies, and decide how to teach the concept in a way that will most benefit my students.  I couldn't believe it when MX went from counting quarters, to counting dollars, to subtracting from 100.  All in a matter of 3 lessons.  REALLY? Subtracting across double zeros to start? What are you people thinking? OK- I mean I kinda get it, subtracting money.. but it just seemed too adventurous to start out with for me.  So this is how we started...

1- Explanation: "Today we are going to begin something new in subtraction where we need to ungroup in order to find the answer.  This can be a very tough thing to understand but if you stick through it, I know you will be able to wrestle and beat this new way of problem solving!  One thing that is really important is that you UNDERSTAND HOW we solve problems like this and WHY.  When you go home at night to do your homework, if you're not sure how to do it, you can ask mom and dad for help- but if they are not doing it the way I am showing you and you don't understand what they are saying, I want you to come back to school with your homework unfinished, yes unfinished, and I will help you understand because the most important thing is that you understand and can explain what we are doing.  You need to know more than the steps to solving these problems and we can learn it together!"

I also sent this note home to parents to let them know: Feel free to save & use as you please (It's not really cute but it gets the point across)

2- Introduction: I gave each student a work mat that had a hundreds, tens and ones column.  Each table group got 1 box of unfix cubes.  First I asked them to make 2 tens.  Then 3 ones.  (How much do we have, value yadda yadda etc).  Then, I told them that I wanted them to subtract 7.  Some of them took 7 off of the 10, left the other 3 intact (in 10's column) and said they got the answer, others didn't know what to do, and a few others decided to break apart their 10.  Yay!  I asked those students to explain what they did and praised them for their wonderful thoughts- it's much more meaningful coming from them than me.  Then we did another problem together just like this, but went through step by step.  Here's a picture of one of my kiddos in action...

We tried this a few more times and they were really getting it! (I have no idea what I'll do when we get to subtracting from 100 or more, I don't have enough unifix cubes for that!!) Tomorrow, I'll introduce the paper & pencil strategy that involves the expanded method and hope that {most} of them will get it. This is my first time teaching this to 19 of my own little babies, so I'm hoping this strategy will prove to be an effective one that will lower both my stress & theirs when it comes to the difficulties of subtraction with ungrouping! (Why the heck can't we just say TRADE!?)


  1. This is so hard! Even in 4th we had to go over it again! This looks great though and they'll get it! We used expanded notation to do it this year, too, which blew their minds (and mine too!)! I sure didn't learn it that way! ;O)

    Collaboration Cuties

    1. You are not kidding! I just don't understand how teaching them to "trade" will confuse them!! It's how I learned it and it makes perfect sense to me. But then again, If you trade, each thing gets something out of it, and in subtracting then 10 (or 100 1,000 etc) gets nothing. They're just getting too technical man! If something works, why change it? :)

  2. Yeah...my Math Ex. book is permanently stuck to my bookshelf :) I just use the common core to guide me and do my own thing. The way they jump around in there drives me c-r-a-z-y.

    Carolina Teacher

    1. Well I'm glad I'm not the only one!! It is seriously insane and makes me go crazy, so I need to do my own thing to keep my sane!! :)

  3. this is stupidest thing I have ever seen
    why cant we just subtract and borrow .cross out
    like I did in 1969???

  4. I am so lost with this. My daughter is in the 3rd grade. And the teacher is jumping into subtracting with ungrouping in the 100s. I'm confused. Half her answers are wrong. Why can't we just go back to borrowing and carrying over with our math. Who is going to use this method in the real world??