Inform the students that they will find differences using the number line model. Make a number line on the floor with numbered carpet squares or by drawing on a shower curtain. Display a subtraction problem, such as 9 – 5 = ___. Ask one student to stand on the number line at 9. Ask the rest of the students which way the student would hop to subtract. When students answer that the student should hop to smaller numbers, have the volunteer hop back 5 spaces. Encourage children to count aloud as each backward hop is made. Describe the action this way:
If you start at 9 and take 5 backward hops, you land on 4.
Allow other students to demonstrate additional subtraction sentences.
After several examples, model how to record counting back on a number line. Display a number line on the board or overhead projector. Write a subtraction sentence, such as 10 – 3. Use a counter to act out this problem, asking students where to place the counter and how many backward hops you should take. After acting this out, record it by circling the number 10 on your number line and drawing 3 backward hops.
Put the children into pairs, and give each pair a counter and an individual number line (or a 12‑inch ruler that they can use as a number line). Have students work in pairs to solve subtraction problems on their number lines and share their answers. Optionally, have students record their action using the Number Line Hopping activity sheet before sharing the difference. After some practice, encourage the students to predict the differences and verify their predictions by moving a counter on the number line.
Discuss the order of numbers in subtraction by asking questions such as, "If I start at 9 and hop backward 5 spaces, will I get the same answer as if I start at 5 and hop backward 9 spaces?" Encourage the students to conclude that the order property, which works for addition, does not work for subtraction.
