Curriculum  /  Math  /  3rd Grade  /  Unit 1: Rounding, Addition, and Subtraction  /  Lesson 13

Rounding, Addition, and Subtraction

Lesson 13

Math

Unit 1

3rd Grade

Lesson 13 of 14

Objective

Solve one-step word problems involving subtraction, using rounding to assess the reasonableness of answers.

Common Core Standards

Core Standards

  • 3.NBT.A.1 — Use place value understanding to round whole numbers to the nearest 10 or 100.
  • 3.NBT.A.2 — Fluently add and subtract within 1000 using strategies and algorithms based on place value, properties of operations, and/or the relationship between addition and subtraction.
  • 3.OA.D.8 — Solve two-step word problems using the four operations. Represent these problems using equations with a letter standing for the unknown quantity. Assess the reasonableness of answers using mental computation and estimation strategies including rounding. This standard is limited to problems posed with whole numbers and having whole-number answers; students should know how to perform operations in the conventional order when there are no parentheses to specify a particular order (Order of Operations).

Foundational Standards

  • 2.NBT.B.7
  • 2.OA.A.1

Criteria for Success

  1. Solve one-step word problems involving subtraction (MP.4).
  2. Use rounding to estimate the answer to a subtraction problem in each of the following cases: 
  • Use rounding to assess the reasonableness of one’s own or someone else’s answer (MP.1).
  • Use rounding when an exact answer is not needed or expected (e.g., the problem says "about" or "approximately").
  1. Determine which estimate is closest to the actual solution and understand why it's closest. 
  2. Understand the effect rounding has on a solution and why (i.e., why one’s estimate is higher or lower than the actual solution). 

Tips for Teachers

  • Students should be familiar with all addition and subtraction situation types from Grade 2. Thus, the focus of the lesson should be on the key advancement – assessing the reasonableness of their answers to these contextual problems.
  • Students may struggle with seeing the usefulness of rounding to assess reasonableness in this and even subsequent lessons, especially since "rounding two numbers before computing can take as long as just computing their sum or difference" (NBT Progression, p. 12). To continue to reinforce its importance, find opportunities to use a mistake as a learning opportunity for how rounding to assess reasonableness can help to identify these mistakes.
  • This lesson provides an opportunity to connect two domains in the grade, 3.OA and 3.NBT, since students will be solving word problems involving subtraction. While students will not solve two-step problems involving addition and/or subtraction (3.OA.8) until Lesson 14, they will use rounding to assess reasonableness of their answers to one-step problems that requires subtraction within 1,000 in this lesson, connecting standards 3.OA.8, 3.NBT.1, and 3.NBT.2.
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Anchor Tasks

Problem 1

Ms. Holcombe buys 850 snacks for Monday and Tuesday. After handing out one snack to each student on Monday, she wants to make sure there will be enough snacks on Tuesday without having to count all of them. Since there are 438 students at Match Community Day, she says, "There are about 400 students at MCD. That means we have about 450 snacks left after handing out Monday’s snack. That’ll be enough for Tuesday!"

a.   Explain why Ms. Holcombe is not correct.

b.   How could Ms. Holcombe have used rounding to estimate more accurately?

c.   What is the exact number of snacks leftover for Tuesday? 

Guiding Questions

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Problem 2

Millie has 403 pennies in her piggy bank. Paulina has 185 pennies in her piggy bank. How many more pennies does Millie have than Paulina? Solve. Then assess the reasonableness of your answer. 

Guiding Questions

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References

EngageNY Mathematics Grade 3 Mathematics > Module 2 > Topic E > Lesson 20 — Application Problem

Grade 3 Mathematics > Module 2 > Topic E > Lesson 20 of the New York State Common Core Mathematics Curriculum from EngageNY and Great Minds. © 2015 Great Minds. Licensed by EngageNY of the New York State Education Department under the CC BY-NC-SA 3.0 US license. Accessed Dec. 2, 2016, 5:15 p.m..

Modified by Fishtank Learning, Inc.

Problem 3

Ms. Dowling’s class knows there are 149 students in Grades 11 and 12, and 317 students at Match High School in total. Three students are trying to figure out about how many students are in Grades 9 and 10 at Match High School. Here’s how each of them computed:

Student Computation
Matt 300 - 100
Beatriz 320 - 150
Alicia 317 - 100

Whose estimate is closest to the actual solution? How do you know?

Guiding Questions

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References

EngageNY Mathematics Grade 3 Mathematics > Module 2 > Topic E > Lesson 20 — Concept Development

Grade 3 Mathematics > Module 2 > Topic E > Lesson 20 of the New York State Common Core Mathematics Curriculum from EngageNY and Great Minds. © 2015 Great Minds. Licensed by EngageNY of the New York State Education Department under the CC BY-NC-SA 3.0 US license. Accessed Dec. 2, 2016, 5:15 p.m..

Modified by Fishtank Learning, Inc.

Problem Set

Answer Keys

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Discussion of Problem Set

  • With your partner, compare your methods of estimation in #1(a) and #2(a). Which was a more precise estimate? If you rounded in the same way, think of another way to estimate. Compare both estimates to the actual answer, and explain why one is more precise than the other.
  • Look at #3. Was your answer reasonable? Why or why not? If your answer wasn’t reasonable, what can you do to fix it? 
  • When do you need to round so that mental math is easy and fast? When do you need to round more precisely?

Target Task

Kathy has a total of $416 in two bank accounts. The amount of money in each account is shown in the table below.

Bank Account A $363
Bank Account B ?

a.   Estimate the total amount of money in Bank Account B.

b.   How much money is actually in Bank Account B?

c.   Is your answer reasonable? Why or why not?

Student Response

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Additional Practice

The Extra Practice Problems can be used as additional practice for homework, during an intervention block, etc. Daily Word Problems and Fluency 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.

Extra Practice Problems

Answer Keys

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Word Problems and Fluency Activities

Word Problems and Fluency Activities

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Lesson 12

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Lesson 14

Lesson Map

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

Topic A: Foundations of Place Value

Topic B: Rounding to the Nearest Ten and Hundred

Topic C: Addition and Subtraction Within 1,000

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