# Knockin It Out of the Park by Subtracting Ten

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## Objective

SWBAT subtract tens within 100 by recognizing how the digit in the tens place is changing.

#### Big Idea

My students need to apply their understanding of place value to properties of addition and subtraction. We continue to build that understanding today by connecting to subtraction facts they already know well.

## Rev Them Up

5 minutes

I will review adding ten with my students while playing a game of, "Cross the Line." I put a strip of masking tape down on the floor long enough that every student will be able to stand behind the line. I ask every student to stand with their toes behind the line. I state a problem with its answer and tell the students if they think I am correct, they should step over the line. If they think I am wrong, they should stay put.

I will state problems like:

• 50+30=80 (step over)
• 30+10=50 (stay put)
• 20+40=60 (step over)

Here you will see my students playing the game.

## Whole Group Interaction

15 minutes

Students must develop a strong understanding of place value to use tens and ones to assist in addition and subtraction of 2-digit numbers. The CCSS want first graders to build the knowledge of place value to be able to complete addition and subtraction with 2-digit numbers. This requires being able add the ones to the ones then the tens to the tens in an equation.  Similar to yesterday's lesson with addition problems, this lesson will allow my students to use their understanding of the base-ten system to solve subtraction problems (1.NBT.C.4). Students use numbers as symbols to represent a problem, solve it, and explain their thinking.

I will use base-ten blocks to assist my students in subtracting tens. Many of my students need to see the concrete representation and will use manipulatives during their independent practice to solve their problems. It is normal for this age group for some students to still be in the concrete operational stage.

I will present several problems to them and use a dry erase board to show the digits. I will have them help me put the correct quantity of blocks on display and then subtract the given amount.

For example:

Students, I need to solve 30-20; how many base ten blocks do I need to start with? (3)

How many do I need to take away? (2 tens)

How many is left? (10)

I will write our answer on the board.

My goal is for my students to identify the simple subtraction that is occurring in the tens place. They have become fluent in subtracting within 10, and I want them to see how subtracting the tens digits to solve this kind of problem simplifies it into something they already know well.

## Independent Practice

5 minutes

Independent practice can sometimes be completed through activities, but there are times you need quality worksheets for students to practice a skill. You do not have to waste time creating a new worksheet, if it already exist. I go to tlsbooks.com to print practice worksheets. Here is the one I chose for this lesson.

This connection will give you 4 different worksheets that build in complexity. I chose the first worksheet and will follow this procedure because the worksheet does contain both addition and subtraction. I introduced addition of tens yesterday and subtraction during the whole group interaction. I want them to practice the skill of subtracting tens first, before they worry about reviewing addition of tens. I will check the subtraction sheet and see if any reteaching is necessary. As I see success, I will staple the addition practice on top of the subtraction sheet and allow them to complete it for review. Here they are completing their independent practice.

## Closing

5 minutes

I will close out today's lesson by having my students find a partner to share what they learned and how they solved their work.

Students I want you to stand up and push your chair in. I will count to 5 aloud. While I am counting, I want you to walk around the room. When I stop counting at 5, you have to have your partner in front of you. Ready 1,2,3,4,5. Okay I want you to tell your partner how you solved your problems today and what you learned about double digit subtraction.