Subtraction Strategies

Demonstrate How Many in the Bag. Show students a bag and tell them it contains nine counters. Remove some and place them on the display. Removing six will be used as an example.

Students should tell how many are in the bag as well as explain how they found their answers. If no one suggests the counting-up strategy, model the process:

1. Point to the 6 counters and ask students to count them.
2. Count up from 6 to 9; 7, 8, 9. Tell students "put 6 in your head." Put up a finger each time you say a number: 7 (one finger), 8 (two fingers), 9 (three fingers).
3. Write a number sentence: 9 − 6 = 3.
4. Check the answer by removing and counting the remaining counters.

Repeat the process one or more times. Each time, begin by removing at least half the counters so that the counting-up process does not become cumbersome or complex. Demonstrate writing a number sentence for each pull.

Play How Many in the Bag. Give each pair of students a bag containing counters with the number of counters indicated on the outside. Explain that they will work in pairs to do the same activity, repeating the process five or six times. They should record a number sentence for each pull on the How Many in the Bag Number Sentences page in the Student Activity Book. Adjust the number of counters in the bags using numbers from 8 to 15 to match the abilities of the students.

As you observe the students working, listen for language which indicates an understanding of the counting-up strategy. Support students who use other ways of thinking to work out subtraction problems. See Figure 2.