The purpose of this activity is for students to connect their understanding of the counting sequence within 1,000 and their understanding of place value to the structure of the number line. In the Launch, students make sense of 3 number lines that have different unit intervals. They may reason about the numbers each tick mark represents by counting by 1, 10, or 100. Other students may notice that there are 10 lengths (unit intervals) and relate this to decomposing a ten or hundred to describe the numbers represented by each tick mark. Throughout the activity, encourage students to make connections between the reasoning they use based on counting and their understanding of place value as they write three-digit numbers and make sense of the structure of the number line (MP7).

• “How did you decide how to label the point?”
• “Looking at the starting and ending numbers, what skip-counting pattern could you use to count the tick marks?”

• Display the number line showing 620–630.
• Invite previously identified students to share how they identified 629.
• Consider asking:
  • “How can you prove that the point represents 629?”
  • “What numbers do the tick marks represent on this number line? How did you count the tick marks?”
• As time permits, invite students to share how they identified other points.

The purpose of this activity is for students to use their place value understanding to locate numbers on a number line. Number lines are given with a starting number, 10 length units marked with tick marks, and an ending number. None of the tick marks are labeled. Students determine the size of the units based on the range of the number line. For example, if the starting and ending numbers are 0 and 100, students may reason that the unit represented by the tick marks is ten because there are 10 tens in a hundred. Once students have determined the unit marked on the number line, they are able to count to find the location of each number. Students may begin to notice that when the two ticks at the right and left of the number line are 100 apart, the individual tick marks count on by 10 and when the two tick marks at the right and left of the number line are 10 apart, the individual tick marks count on by 1 (MP8).

• Display the number line showing 900–1,000.
• Invite a student to demonstrate their strategy of trial and error to determine how to label the tick marks.
• Invite another student to demonstrate their strategy of reasoning about the starting and ending numbers. (I know the difference between 900 and 1,000 is 100 and there are 10 length units. Each one must be 10 because there are 10 tens in a hundred.)
• “What is the same about how they decided the unit on this number line? What is different?” (They both counted to see how many tick marks are on the number line. _____ tried counting by 1 and then 10, but _____ counted by 10 right away.)