Lesson 14 Think Before You Subtract

- Let’s think about decomposing before we subtract.

Warm-up Which One Doesn’t Belong: Blocks and Blocks and Blocks

Which one doesn’t belong?

Activity 1 Agree to Disagree

Tyler and Clare are subtracting by place to find the value of $244 - 67$ . Tyler says he will decompose before he starts. Clare says she agrees.

The diagrams show each student’s first step.

Tyler:

Base ten diagram. 1 hundred, 14 tens, 4 ones.

Clare:

Base ten diagram. 2 hundreds, 3 tens, 14 ones.

What is the same about Tyler and Clare’s diagrams? What is different?
Work together to complete Tyler’s way and Clare’s way to find the value of $244 - 67$ .
What do Tyler and Clare’s diagrams look like after the last step? What is the same about these diagrams? What is different?

Activity 2 Sort and Subtract

Here is a base-ten diagram for 341.

Base ten diagram. 3 hundreds, 4 tens, 1 one.

Andre wants to use diagrams and subtract by place to find the value of $341 - 68$ . He says he will decompose a ten and a hundred to subtract. Why do you think he said that?
Andre only wants to use a diagram to subtract by place if he will decompose a unit. Help Andre sort the expressions into groups. If you are not sure, use base-ten blocks or a diagram to help.
$\begin{matrix} 599 - 66 \\ 633 - 55 \\ 457 - 45 \\ 905 - 42 \\ 514 - 26 \end{matrix}$
$\begin{matrix} 449 - 88 \\ 237 - 29 \\ 735 - 72 \\ 693 - 63 \\ 387 - 44 \end{matrix}$
$\begin{matrix} 346 - 78 \\ 321 - 34 \\ 645 - 87 \\ 866 - 58 \\ 277 - 65 \end{matrix}$
decompose 2 units
decompose 1 unit
do not decompose
Find the value of 1 expression from each group. Show your thinking.

Practice Problem

Problem 1

Jada is finding the value of $571 - 385$ . She says that she can take the ones from the ones, tens from the tens, and hundreds from the hundreds, with no decomposing. Do you agree with Jada? Explain your reasoning.