The purpose of this activity is for students to analyze two different subtraction methods that are based on place value and connect the methods to equations. In previous lessons, students analyzed base-ten drawings like Lin’s where a student recognizes that a ten needs to be decomposed before they draw the blocks. Clare’s drawing represents a student who mentally subtracts tens from tens before drawing and then considers decomposing units. Students discuss how each method works and deepen their understanding of the properties of operations (MP7).

• Invite students to share the group of equations that match Lin's work and Clare’s work.
• “How are these methods the same?” (See Student Responses, second problem.)
• “How are the methods different?” (See Student Responses, third problem.) 
• “Did Clare find the value of ? What did you learn about her method?” (Yes, she did because she did take away tens. I learned you can take away tens first and it doesn’t change the difference.)

The purpose of this activity is to analyze a subtraction method that is based on place value and connect it to equations. Students analyze the method and explain why they think it best matches one of the methods they saw in the previous activity. Then they practice subtraction using any method that makes sense to them. Monitor for different methods to share in the Synthesis, including students who show ways to subtract ones from ones first and those who subtract tens from tens first. 

• “What did you do first to find the value of _____? What did you do next?”
• “How could you use labels or equations to help keep track of your steps?”

• Invite 1–2 students to share their method for each difference. 
• Consider asking or inviting students to ask questions of students who share:
  • “What did you do first?”
  • “Why did you choose this representation?”
  • “How is your method like _____’s method?”