Perimeter vs. Length Lab

TIMS Task

Encourage students to share their strategies. Some students may use strategies such as using doubles, making tens (hundred), or using tens (hundred). The addition doubles, e.g., 80 + 80 = 160 and 60 + 60 + 120, can be used to learn the subtraction “halfdoubles” as well: 160 − 80 = 80 and 120 − 60 = 60. These facts can then be used to figure out near doubles, such as 130 − 60 = 70 and 150 − 70 = 80.

Knowing the addition facts that have a sum of 100 can be helpful in solving a problem like 130 − 60 = 70 as well. To solve 130 − 60, a student might count up and make a hundred—60 + 40 = 100; then 100 + 30 = 130. Thus 40 + 30, or 70, is the answer.

1. 30
2. 70
3. 80
4. 150
5. 80
6. 90

7. Possible response: I know 18 − 9 = 9, so 180 − 90 = 90