Lesson 7 Compare Methods for Subtraction

    • Let’s compare subtraction methods.

Warm-up True or False: How many Tens? How many Ones?

Decide if each statement is true or false. Be prepared to explain your reasoning.

Activity 1 Different Ways to Decompose

Diego and Elena drew base-ten diagrams to find the value of .

Diego

Base ten diagram. 8 tens and 2 ones. One of the tens is crossed out with an arrow pointing to a circle with ten ones inside. The two ones are crossed out and 7 ones inside the circle are crossed out.

Elena

Base ten diagram. 7 tens and 12 ones. 9 ones are crossed out.

Problem 1

Compare Diego’s work to Elena’s.

  1. What is the same?

  2. What is different?

Problem 2

Tyler used equations to show his thinking.



Diego says Tyler’s work matches his diagram.
Elena says Tyler’s work matches her diagram.

Who do you agree with? Explain.

Activity 2 Introduce Target Numbers: Subtract Tens or Ones

We are going to learn a new way to play Target Numbers. You and your partner will start with 99 and race to see who can get closest to 0.

Practice Problem

Problem 1

Here is Mai’s work with a subtraction expression.

Base ten diagram. 8 tens and 5 ones. One of the tens is crossed out. An arrow points to a circle with 10 ones inside. The 5 ones are crossed and 2 ones inside the circle are crossed out.

  1. What subtraction expression does Mai’s diagram show?

  2. What is the value of the expression?

  3. Use Mai’s method to find the value of .