standard_description: Given a two-digit number, mentally find 10 more or 10 less than the number, without having to count; explain the reasoning used.
problem_activity: Narrative
The purpose of this activity is for students to learn stage 5 of the Target Numbers center. Students subtract two-digit numbers to get as close to 0 as possible. For the introduction of this stage, invite students to count by 10 to 100 and represent their count with base-ten blocks. Then, they take turns rolling the 3 cubes to get a number to subtract. They choose one of the numbers on the cubes to represent the tens and a different number to represent the ones. Students subtract their tens and ones from the starting number. The difference becomes the first number in the next equation. The player who gets closest to 0 in 6 rounds, without going below 0, is the winner.
Required Materials
Materials to Gather
Base-ten blocks
Number cubes
Materials to Copy
Target Numbers Stage 5 Recording Sheet
Required Preparation
Each group of 2 students will need 3 number cubes.
Each group of 2 students will need base-ten blocks to represent at least 20 tens and 18 ones
.
Launch
Groups of 2
Give each student a copy of the recording sheet.
Give each group 3 number cubes and access to base-ten blocks.
“We are going to learn a new way to play Target Numbers. You and your partner will start with 100 and race to see who can reach a number less than 10 first.”
“Instead of using cards to decide whether to take away tens or ones, you will use number cubes to create a two-digit number and then subtract that number.”
“First, represent 100 with base-ten blocks."
As needed, invite students to count by 10 to 100 using the base-ten blocks or invite student to share how they might represent 100 with the blocks.
"When it’s your turn, roll all 3 number cubes. Pick 1 number to represent the tens and one number to represent the ones. Then show the subtraction with your blocks and write an equation on your recording sheet.”
“Take turns rolling and subtracting until the first person reaches a number less than 10.”
As needed, demonstrate a round with a student volunteer.
Activity
10
–
15 minutes: partner work
Activity Synthesis
Display equation:
\(73 - \underline{\hspace{1 cm}} = \underline{\hspace{1 cm}}\)
Draw or display number cubes showing 4, 2, and 5.
“What two-digit numbers could you create with this roll that would make you need to decompose a ten to subtract? How do you know?” (Sample response: 24 because if you subtract by place, there are only 3 ones, so you would need to decompose to take away 4 ones.)
“What two-digit numbers could you create that wouldn’t require decomposing a ten? How do you know?” (Sample response: 52 because if you subtract by place there are 3 ones and you can take away 2 ones from 3 ones.)
“What number would you create to try to get below 10 the fastest? Explain. (54 because it’s the number that lets you take away the most tens and ones.)


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