Adding and Subtracting Multiples of Ten

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SWBAT build a two digit number with groups of tens and ones. SWBAT add or subtract a multiple of 10 from a number.

Big Idea

Students will apply the idea of adding or subtracting ten to a number by adding or subtracting multiple groups of ten from a number.

Warm Up

10 minutes

I start by having a class sit in a circle on the carpet.  I want to continue to focus on counting by tens off the decade.  I will ask a student to help me model the count for the class.

"We are going to count by tens again today.  We will start at 5 and count to 95 and then count from 95 back to 5.  Today, we will do it as partners.  We will all be working on the same count, but will be counting in teams of two. Who can help me model this for the class?"  In this case students are mentally adding or subtracting a group of ten from a given number (CCSS.MATH.CONTENT.1.NBT.C.5)

There is a video in the section resource that models this discussion.

Note:  Some students may still need to use a number grid or line to perform this activity.

Introduction of Roll A Number

15 minutes

Advanced Preparation:  Students will each need a set of mini ten frame cards, a spinner, and a recording sheet.  Each of these can be found in the section resource.  You will also need the tens and ones dice from a place value dice set (one set for each student).  

I then transition from the partner counting to the introduction of the game.  

"I want to teach you a new activity that focuses on adding and subtracting groups of ten to/from a number.  The activity is called Roll A Number.  You will do this activity on your own.  You start by rolling the 10's dice and the 1's dice and then state the number.  You will then build that number with your tens frame cards.  Then you will spin the spinner and either add or subtract a multiple of ten to/from the number.  Then you perform the action and record your equation on the recording sheet."

There is a video in the section resource of this introduction.

Note:  For students who are still working on the initial concept of adding or subtracting ten, they can use the +10/-10  spinner and the adapted recording sheet.  The spinner is on the same sheet as the regular spinner and the adapted sheet is in the section resource.

Station TIme

30 minutes

The students now work on this activity on their own.  As they are working you will want to see how they are adding or subtracting multiples of ten.  You want to see who can just change the tens place, who needs to use the cards, and if anyone still needs to count by ones.

There are two videos in the section resource.  The first one is of a student working on the activity.  You can hear the type of questions that I ask her as she is playing.  

The second video is of a student who is already secure with this concept (using numbers through 900).  I have given him the whole set of place value dice and asked him to work on adding and subtracting multiples of ten from thousands numbers.  This way I can see how he is writing 4 digit numbers and if he can us this knowledge with any number.  

In this case, the students are meeting the CCSS goal of "adding within 100, including adding or subtracting a two-digit number and a one-digit number, and adding a two-digit number and a multiple of 10, using concrete models or drawings and strategies based on place value concepts."

Some students will also be  able to mentally find 10 more or 10 less than the number, without having to rely on models or counting.(CCSS.MATH.CONTENT.1.NBT.C.4 CCSS.MATH.CONTENT.1.NBT.C.5 & CCSS.MATH.CONTENT.1.NBT.C.6)

Lesson Wrap Up

10 minutes

I want to end this lesson with a discussion about what students notice when they are adding or subtracting tens from a number.  I want to continue to focus on the fact that the number change because the groups of ten changes.  I also want to reinforce the idea that a written number is a representation of the groups of tens and ones.

"Lets say I start at 64 and then add another group of 10.  How would I write that?  . . . You're right it would be 74.  Why do we now write 74?  What would be next?"

I continue this conversation with a few more examples of adding ten 10 and then I also do an example with subtraction.  

During this discussion, students are "making conjectures and build a logical progression of statements to explore the truth of their conjectures." The students are also "justifying their conclusions, communicating them to others, and responding to the arguments of others" (CCSS.MATH.PRACTICE.MP3).

Continued Practice

5 minutes

I will ask the students to meet me on the carpet and hand out their sheet for today's Mad Minute exercise.  This routine was introduced in a previous lesson.  Please check out the link to get a full overview of this routine.

I want to really focus on fact fluency and build upon the students ability to solve within ten fluently (CCSS.MATH.CONTENT.1.OA.C.6).  I am going to use the Mad Minute Routine.  This is a very "old school" routine.  However, I truly feel students need practice in performing task for fluency in a timed fashion.  Students need to obtain fact fluency in order to have success with multiplicative reasoning.  Students who don't gain this addition fact fluency by the end of 2nd grade tend to struggle with the multiplicative reasoning in third.  Having this fluency also allows them to work on more complex tasks because the have the fact recall to focus on the higher level concepts.