The purpose of this activity is for students to connect a subtraction method based on place value to representations on the number line. Students compare representations of a subtraction method using base-ten diagrams, equations, and number lines (MP2). They notice that, just like with base-ten blocks, they can count by tens first or by ones first on the number line to represent subtracting.

• “Can you tell me more about your number line? How did you decide where to start?”
• “How does the number line connect to your base-ten diagram?”

• Invite previously identified students to share.
• Display or record their methods.
• Consider asking each student:
  • “How did you decide where to start, how many jumps to make, and the length of each jump?”
• “How are these methods the same? How are they different?” (They started at 58. Some show subtracting tens first, and some show subtracting ones first. Some show subtracting the value of all the tens or all the ones. Some show subtracting each ten or each one.)
• “How does the number line help you see how these methods are the same?” (It helps you see that it doesn't matter if you subtract tens first or ones first. They all show subtracting 24.) 

The purpose of this activity is for students to represent addition and subtraction within 100 on a number line. Students make connections to strategies based on counting on or back by place. The numbers in each subtraction equation are designed to elicit methods that do not require students to explicitly decompose a ten. For example, when finding the value of , students may first add on to make a ten (), then add on more tens to reach the total (). Others may see they can count back 2, then subtract the tens. In the Synthesis, students share their thinking and discuss how the number line helps them see how they can use what they know about the structure of counting sequences and what they know about tens and ones to add and subtract (MP7).

• Invite 2–3 previously selected students to share.
• Consider asking each student:
  • “How did you decide where to start?”
  • “How did you decide how much to add/subtract first?”
  • “How does your number line show the value of the difference?”
• “What other questions do you have about _____’s number line?”
• “How does the number line help you make sense of _____’s method?”