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 solve two story problems involving adding or subtracting multiples of 10. Students are presented with a familiar context from a previous lesson in which students counted cubes in different bags. The quantities of cubes are described using representations that students are familiar with, and students add or subtract in a way that makes sense to them. The familiar context and representations are used to help students make sense of adding and subtracting tens. In the synthesis, students compare and connect the different representations used for each problem (MP2).
MLR6 Three Reads.
Keep books or devices closed. To launch this activity, display only the problem stem, without revealing the question. “We are going to read this story problem three times.” After the 1st Read: “Tell your partner what happened in the story.” After the 2nd Read: “What are all the things we can count in this story?” Reveal the question. After the 3rd Read: “What are different ways we can solve this problem?”
Advances: Reading, Representing
Required Materials
Materials to Gather
Connecting cubes in towers of 10 and singles
Double 10-frames
Launch
Groups of 2
Give students access to connecting cubes in towers of 10 and singles and double 10-frames.
Activity
Read the task statement.
7 minutes: independent work time
3 minutes: partner discussion
Monitor for students who show:
towers of 10
base-ten drawings
__ tens and __ tens
expressions or equations
Student Facing
Jada is counting collections of cubes.
In Bag A there are 30 cubes.
In Bag B there are 2 towers of 10.
How many cubes are in the two bags all together?
Show your thinking using drawings, numbers, or words.
Tyler is counting a collection of cubes.
In Bag C there are 7 towers of 10.
He takes 40 cubes out of the bag.
How many cubes does he have left in the bag?
Show your thinking using drawings, numbers, or words.
Student Response
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Advancing Student Thinking
If students attempt to add or subtract using single cubes (or representations of single cubes), consider asking:
“How are you planning to find out how many cubes there are now?”
“How could you use what we've learned about tens to solve this problem?”
Activity Synthesis
Invite previously identified students to share their representations.
“How are these representations the same? How are they different?”


aligns: no