The purpose of this activity is for students to compare three-digit numbers and find the value of their difference. Students used the number line to compare three-digit numbers in an earlier unit. They build on this understanding as they find the difference between 2 three-digit numbers that are within 10 of one another on the number line. Students notice that when the numbers in a subtraction expression are close together, they can count on or count back to find the value of the difference (MP7, MP8).

• Invite previously identified students to share how they found the value of .
• As time permits, invite students to share their comparisons and how they found the value of each difference.
• “What did you notice about the numbers when you located them on the number line?” (They were close together. It was easy to see a way to just count from one number to the other.)

The purpose of this activity is for students to use what they know about counting within 1,000 to make sense of number lines that show counting on or counting back by 10 or by 100. This work helps build fluency with counting within 1,000, and connects to an upcoming lesson where students add and subtract multiples of 10 or 100, using equations.

• “What digit is changing? Is it going up or down?”
• “How can that help you figure out the count?”

• Invite a student to share their reasoning for _____, _____, 332, 342, 352.
• “How did you figure out the starting number?” (I saw that it was going up by 10, so I knew I had to count back by 10 to find the starting numbers.)
• Record responses with an open number line and equations.