Domain

Number and Operations in Base Ten

Cluster

Use place value understanding and properties of operations to add and subtract.

Standard(s)

NC.2.NBT.5  Demonstrate fluency with addition and subtraction, within 100, by:

• Flexibly using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction.

• Comparing addition and subtraction strategies, and explaining why they work.

• Selecting an appropriate strategy in order to efficiently compute sums and differences.

Materials

Pencil, Paper

Task

Colby and Audrey both solved 44-29 correctly, but they did it in different ways.  Explain why each strategy works.  How are the strategies the same?

Not Yet Proficient

·         Explanation is non-existent

Sample Answers:

·         Audrey used a number line to count up from 29.  She made one jump to get to 30, an easier number work with.  Then she jumped to 10 to get 40 and jumped one at a time to get to 44.  To get her answer, she added all her jumps for 15.

·         Colby decomposed 29 (20 + 4 + 5) to make it easier to subtract.  He subtracted 20 from 44 to get 24.  It was easy to take 4 from 24 to get 20.  Subtracting 5 from 20 gave him 15. 

·         Both students used friendly numbers (multiples of 10) to make solving easier. 

Progressing

·         Student explains one of the strategies with accurate details.

Meets Expectation

• Students explain both strategies with accurate details.

• Students can give a similarity between the two strategies.