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 practice adding a one-digit number and a two-digit number in a way that makes sense to them. Students may choose to use methods shared in the previous activity. Give half of the class one-digit numbers and the other half two-digit numbers. Students pair up to create a sum with a two-digit number and a one-digit number. In some problems, the sum will require composing a ten when adding ones to ones. Monitor for students who share ways they make a new ten with connecting cubes or drawings to share in the synthesis.
When students choose a method to add based on the pair of numbers they are adding, they make use of structure and apply regularity in reasoning (MP7, MP8). This could mean counting on when the 1-digit number is small, adding the ones when there is no new ten, and adding on to make a ten if needed.
MLR8 Discussion Supports.
Synthesis: For each method that is shared, invite students to turn to a partner and restate what they heard using precise mathematical language.
Advances: Listening, Speaking.
Required Materials
Materials to Gather
Connecting cubes in towers of 10 and singles
Materials to Copy
Add Em' Up Cards (2-digit and 1-digit numbers to 100)
Required Preparation
Create a set of cards from the blackline master for the class.
Launch
Groups of 2
Give students access to connecting cubes in towers of 10 and singles.
Give half of the students one-digit number cards and half of the students two-digit number cards.
Display the student workbook page with the practice round.
“Each person has a card with a number on it. You will find a partner and find the sum of your two numbers. One student should have a one-digit number and the other should have a two-digit number. Let's do one round together.”
Invite one student with each kind of card to share their number.
“Find the sum of the numbers. Show your thinking using drawings, numbers, or words. Then write an equation that shows the value you found.”
2 minutes: independent work time
Share an equation that could be used.
“Now we will all play a few rounds.”
Activity
“Find a partner and find the sum of your numbers.”
5 minutes: partner work time
“Switch cards with your partner and find a new partner.”
Repeat as time allows.
Monitor for students whose sum:
does not require composing a new ten.
requires composing a new ten when adding ones and ones.
Student Facing
Practice Round:
Show your thinking using drawings, numbers, or words.
Equation: __________________________________
[IMG]
Round 1:
Show your thinking using drawings, numbers, or words.
Equation: __________________________________
Round 2:
Show your thinking using drawings, numbers, or words.
Equation: __________________________________
Round 3:
Show your thinking using drawings, numbers, or words.
Equation: __________________________________
Round 4:
Show your thinking using drawings, numbers, or words.
Equation: __________________________________
Round 5:
Show your thinking using drawings, numbers, or words.
Equation: __________________________________
Student Response
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Activity Synthesis
Invite previously identified students to share.
After each student shares, ask “Why did you choose this method?” (Since I was adding only 3, I could count on quickly. The number 9 would take too long to count on so I broke it apart to make a new ten and then added the rest.)


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