Add and Subtract with length.

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The students will be able to measure the length of an object twice, using length units of different lengths for the two measurements; describe how the two measurements relate to the size of the unit chosen

Big Idea

SWAT add and subtract problems involving length.

To Begin

20 minutes

Can someone tell me what we have learned about inches so far? 

Student Responses:

How to place an object to be measured.

We start measuring by placing an object on zero.

we can use other objects to measure.

We can compare two objects.

Todd's puppy was 10 inches tall when he went to the vet last month. Today, the puppy is 13 inches tall. How many inches taller is the puppy now?


  What is the problem asking us to do?

How many inches taller is the puppy now?

What information do the problem give us to help solve the problem?

It tells us that the puppy was 10 inches tall at first, but now it is 13 inches tall.

Are there any key words?

Yes! Taller is  is telling us to subtract to find the missing number.

To build on students’ prior knowledge:

I note that, the length of another object can be added or subtracted from the length of another object. After that, I model aloud what questions to ask as I solve the problem. I wanted students to learn problem-solving strategies to assist them in solving mathematical equations using inches.

Teacher Talk:

Todd's puppy was 10 inches tall. He is now 13 inches tall.

Subtract 10 inches from 13 inches, to find how many inches taller the puppy is now.

13 - 10 = 3

So, Todd's puppy is 3 inches taller.


Both of these are strategies that have been introduced in prior lessons and will help to scaffold new learning. My goal is for students to begin to rely on what they know about length and addition to help them solve two-step word problems.

 We will be focusing on the following Mathematical Practices in this lesson:

MP.2. Reason abstractly and quantitatively.

MP.3. Construct viable arguments and critique the reasoning of others.

MP.5. Use appropriate tools strategically.

MP.6. Attend to precision.

MP.7. Look for and make use of structure



20 minutes

Material: How To Document.docx

I invite students to sit with me on the carpet. I go over the objective for today.  (SWBAT add and subtract length.)  Stating the objective gives the students some idea of how their learning is going to be measured. Unless your objectives are measurable in some manner, there is no way that you can produce the evidence necessary to show that the objectives were in fact met.

 I continue by writing a word problem on the board.  I ask students to read the problem aloud with me. 

  Students and Teacher Talk:

Example 2:

Morgan threw a football 10 meters. Logan threw a football 12 meters.

How far did the girls throw the football in all?


Use n to stand for the total number of meters.

To find how far the girls threw the football in all, add 10 meters to 12 meters.

10 + 12 = n

 Probing questions are good to focus students thinking on the intended purpose of this lesson. 

What is the problem asking us to do?

What information to do we know so far?

Are there any clue words?

What math operation are we using? How do you know?

Can you explain how you solved?

My goal is to assist the students only if needed.  I want them to be independent problem-solvers that rely on what they know in order to solve equations.  I ask students to repeat the objective of this lesson, and to discuss any misconceptions they may have.  (If students are having any misconceptions review circling clue words, and basic addition and subtraction facts.) I ask students to return to their seats, so they can fill out a question card about the problem we solve.

I tell the students that they will be working independently to solve equations. 

 Student Sample



20 minutes

Now that students are back at their seats, they will begin to work independently on solving addition and subtraction problems involving length.  I explain that they will read several word problems and think of ways to solve them.  In addition to explaining I want you guys to become comfortable with explaining how you solve. Therefore, each of you have a problem solving  sheet to write out each step.  I point out that it is not necessary for you all to have the same strategy, but you must be able to explain your method.


While students are working I circle the room to provide assistance when needed.  I ask random students to explain how they determine their answers. Students explain the steps they used, clue words they circled, and whether or not they used arrays to help them solve their answers.(Click on explanation to see details of how students should be able to explain their answers)  I take anecdotal notes along the way to keep track of students strengths and weaknesses. 

Student Sample