Lesson 9 The Difference Between Numbers

    • Let’s represent ways to subtract on the number line.

Warm-up Number Talk: Add To

Find the value of each expression mentally.

Activity 1 Add or Subtract

  1. What number makes this equation true?

    Represent your thinking on the number line.

  2. What number makes this equation true?

    Represent your thinking on the number line.

  3. What number makes this equation true?

    Represent your thinking on the number line.

Activity 2 Different Ways to Find the Difference

Problem 1

Elena had a length of string that was much too long for her project. The string was 65 inches long. Elena cut off 33 inches. How long is the string now?

Choose 2 number lines that show a way to find the length of Elena’s string.

  1. Number line. Scale 0 to 100 by 5's. Arrow from 33 to 65.
  2. Number line. Scale 0 to 100 by 5's. Arrow from 65 to 98, labeled 33. 
  3. Number line. Scale 0 to 100 by 5's. Evenly spaced tick marks. Arrow starts at 65, ends at 32.

Problem 2

Han had 87 inches of string. He cut off 85 inches of it. How much string does he have left?

  1. Write an equation to represent the problem with a ? for the unknown.

  2. Find the number that makes the equation true.

  3. Represent your thinking on the number line.

Problem 3

Find someone who used a different method.

Show their method on the number line.

Practice Problem

Problem 1

  1. Explain or show how each number line represents the value of .

    Number line. Scale 0 to 50 by 5's. Evenly spaced tick marks. Arrow starts at 41, ends at 47.
    Number line. Scale 0 to 50 by 5's. Evenly spaced tick marks. Arrow starts at 47, ends at 6.

  2. Which method do you prefer to calculate ?