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).

Engagement: Provide Access by Recruiting Interest. Optimize meaning and value. Invite students to represent the jumps/movement on the number line, using an animal such as a frog, a rabbit, or a grasshopper. Focus on the minimal jumping the animal does when the numbers are close on the number line.
Supports accessibility for: Conceptual Processing, Attention

If students find the difference by subtracting the second number from the first using base-ten blocks or base-ten diagrams, consider asking:

• “¿Cómo encontraron el valor de la diferencia?” // “How did you find the value of this difference?”
• “¿Estos números son cercanos o lejanos en la recta numérica? ¿Cómo puede ayudarte eso a pensar en una forma diferente de encontrar la diferencia?” // “Are these numbers close together or far apart on the number line? How could that help you think about a different way to find the difference?”

• 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.
• “¿Qué observaron sobre los números cuándo los ubicaron en la recta numérica?” // “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.

• “¿Qué dígito está cambiando? ¿Está aumentando o disminuyendo?” // “What digit is changing? Is it going up or down?”
• “¿Cómo puede ayudarte eso a descifrar el conteo que se está haciendo?” // “How can that help you figure out the count?”

• Invite a student to share their reasoning for _____, _____, 332, 342, 352.
• “¿Cómo descifraron cuál era el primer número?” // “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.