options: Given a two-digit number, mentally find 10 more or 10 less than the number, without having to count; explain the reasoning used.

Add within 100, including adding a two-digit number and a one-digit number, and adding a two-digit number and a multiple of 10, using concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; relate the strategy to a written method and explain the reasoning used. Understand that in adding two-digit numbers, one adds tens and tens, ones and ones; and sometimes it is necessary to compose a ten.

Subtract multiples of 10 in the range 10-90 from multiples of 10 in the range 10-90 (positive or zero differences), using concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; relate the strategy to a written method and explain the reasoning used.
problem_activity: Narrative
The purpose of this activity is for students to use their understanding of place value to make an equation true when a digit in a two-digit number is “missing.” Students consider the values of the digits in order to justify their thinking. Students may use connecting cubes in towers of 10 and singles, create drawings, or write about their reasoning.
MLR8 Discussion Supports.
Prior to solving the problems, invite students to make sense of the situations and take turns sharing their understanding with their partner. Listen for and clarify any questions about the context.
Advances: Reading, Representing
Action and Expression: Internalize Executive Functions.
Invite students to decide with their partners what tools they will use to find the missing digit. Allow time for students to organize tools.
Supports accessibility for: Organization, Attention
Required Materials
Materials to Gather
Connecting cubes in towers of 10 and singles
Launch
Groups of 2
Give students access to connecting cubes in towers of 10 and singles.
Activity
Read the task statement.
10 minutes: partner work time
Student Facing
This equation is true.
[IMG]
What digit is under the smudge?
Show your thinking using drawings, numbers, or words.
This equation is not true.
[IMG]
Show why it is not true using drawings, numbers, or words.
Student Response
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Activity Synthesis
Invite students to share how they determined the missing digit in the first equation.
Invite students to share how they know the second equation cannot be true.


correct_option_index: 1