Curriculum  /  Math  /  2nd Grade  /  Unit 2: Adding and Subtracting Within 100  /  Lesson 12

Adding and Subtracting Within 100

Lesson 12

Math

Unit 2

2nd Grade

Lesson 12 of 21

Objective

Use properties of operations to subtract within 100.

Student-Facing

I can make friendly numbers to subtract.

Common Core Standards

Core Standards

  • 2.NBT.B.5 — Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction.
  • 2.NBT.B.9 — Explain why addition and subtraction strategies work, using place value and the properties of operations. Explanations may be supported by drawings or objects.

Foundational Standards

  • 1.NBT.C.4
  • 1.NBT.C.5
  • 1.NBT.C.6
  • 1.OA.B.3
  • 1.OA.B.4

Criteria for Success

  1. Subtract by decomposing the subtrahend into tens and ones and subtract in parts. 
  2. Use constant difference to subtract when a number is close to a benchmark number. 
  3. Add the constant to both the minuend and subtrahend and then subtract to find the difference. 

Tips for Teachers

  • This lesson allows students to think flexibly about the numbers in their subtraction situations. They are introduced to two strategies to help make their subtraction easier. 
    • The first is to decompose the subtrahend, or the part that is being subtracted from the whole, into workable chunks to subtract. This often involves subtracting multiples of tens and ones in ways that gets students to benchmark numbers. 
    • The second way is for students to use the idea of constant difference. If the subtrahend is close to a benchmark number, they can add that amount to both the minuend and the subtrahend to make it a situation where they are subtracting a friendly number, in these cases a multiple of ten. In Anchor Task 2 it is helpful to have students physically subtract using a different method to see that using constant difference does work. Be sure that students are not just adding the constant to one number as that changes the problem completely.
  • As a reminder, throughout the unit you may notice that we use the phrase friendly or benchmark numbers. We are referring to multiples of ten that are typically easier to work with when adding and subtracting. In student-facing work we often call these "friendly" numbers. In teacher-facing work we often refer to it as "benchmark" numbers. You may also have heard them referred to as landmark numbers as well. Feel free to stick with whatever language you feel comfortable with with your students. 

Warm Up

5-10 minutes

Unlock warm-up activities that help you engage every student and prepare them for the day’s lesson with a Fishtank Plus subscription.

Anchor Tasks

15-25 minutes

Problem 1

Malcolm showed his thinking as he solved 33 - 14 below. 

33 - 14 = ?

33 - 10 = 23

23 - 3 = 20

20 - 1 = 19

a.   What do you notice? What do you wonder about this strategy?

b.   Use similar thinking to solve the following problems.

  1. 27 - 13 =
  2. 54 - 29 =
  3. 62 - 31 =

Purpose

Create a free account or sign in to access the Purpose for this Anchor Problem.

Upgrade to Fishtank Plus to view Facilitation Guidance and Sample Student Response for this Anchor Task.

Problem 2

Karla solved 61 - 14 = by making friendly numbers to subtract. 

61 - 18 =

Solve the following by using Karla's strategy.  

a.   31 - 19 =

b.   53 - 27 =

c.   44 - 38 =

Purpose

Create a free account or sign in to access the Purpose for this Anchor Problem.

Upgrade to Fishtank Plus to view Facilitation Guidance and Sample Student Response for this Anchor Task.

Problem 3

Choose a strategy to subtract. 

a.   70 - 28 =

b.   55 - 24 =

c.   61 - 39 =

d.   84 - 41 =

e.   100 - 88 =

Purpose

Create a free account or sign in to access the Purpose for this Anchor Problem.

Upgrade to Fishtank Plus to view Facilitation Guidance and Sample Student Response for this Anchor Task.

Problem Set

15-20 minutes

Target Task

5-10 minutes

Solve any way. Explain your strategy.

a.   77 - 28 =______

b.   81 - 39 =______

Student Response

Additional Practice

Daily Word Problems, Warm Ups, and Center Activities are aligned to the content of the unit but not necessarily to the lesson objective, therefore feel free to use them anytime during your school day.

Word Problems and Fluency Activities

Next

Relate subtraction with place value drawings to the standard algorithm.

Lesson 13
icon/arrow/right/large

Lesson Map

A7CB09C2-D12F-4F55-80DB-37298FF0A765

Topic A: Adding & Subtracting with Multiples of Ten

Topic B: Strategies for Adding within 100

Topic C: Strategies for Subtracting within 100

Topic D: Working within 100 and Categorical Data

Request a Demo

See all of the features of Fishtank in action and begin the conversation about adoption.

Learn more about Fishtank Learning School Adoption.

Contact Information

School Information

What courses are you interested in?

ELA

Math

Are you interested in onboarding professional learning for your teachers and instructional leaders?

Yes

No

Any other information you would like to provide about your school?

We Handle Materials So You Can Focus on Students

We Handle Materials So You Can Focus on Students

We've got you covered with rigorous, relevant, and adaptable math lesson plans for free