# 1.OA.C.6

Add and subtract within 20, demonstrating fluency for addition and subtraction within 10. Use strategies such as counting on; making ten (e.g., 8 + 6 = 8 + 2 + 4 = 10 + 4 = 14); decomposing a number leading to a ten (e.g., 13 – 4 = 13 – 3 – 1 = 10 – 1 = 9); using the relationship between addition and subtraction (e.g., knowing that 8 + 4 = 12, one knows 12 – 8 = 4); and creating equivalent but easier or known sums (e.g., adding 6 + 7 by creating the known equivalent 6 + 6 + 1 = 12 + 1 = 13).

Click here to learn about the process used to unpack the standards and create learning targets.

## Summary:

Students need a variety of strategies for solving addition and subtraction problems through 20. Students also need to be fluent with any sums or differences within 10. These strategies will provide students with a variety of choices when deciding the best way to solve problems. This standard brings together all of the previous strategies and operations to apply in any given addition or subtraction situation. Fluency within 10 allows sums and differences within 20 to be solved quickly, as students can decompose larger numbers as needed. For example, 13 - 4 can be thought of as 13 - 3 - 1 = 10 - 1 = 9.

## Understanding the Standard:

• Teachers should guide students through a wide variety of methods for solving addition and subtraction problems. While the correct answer is important, students need to feel comfortable in solving for the answer in the way that makes the most sense to them while also having knowledge and practice with other strategies.
• Students need to be taught how to communicate their reasoning for choosing a specific strategy. For example, students can show how they solve 6 + 7 by first changing it to 6 + 6 + 1 (which demonstrates the decomposition of 7 into 6 and 1) followed by 12 + 1 (fluency of knowing 6 and 6 is 12) and then 12 + 1 = 13, which they relate to counting on by one from twelve to get 13.
• Additionally, they should be encouraged to justify their reasoning in a variety of manners such as written and visually with drawings, models and pictures.

## Questions to Focus Instruction:

• Have students been exposed to a wide range of problem-solving tools that go beyond counting fingers and using a number line?
• Can students communicate their reasoning for choosing a specific strategy for problem-solving?
• Can students use the strategy of counting on?
• Can students use the strategy of making a ten?
• Can students use the strategy of decomposing a number to quickly solve an addition or subtraction problem?
• Can students use a related addition fact to solve a subtraction problem?
• Can students create easier sums within a problem to solve a large problem?

## Skills

Prior to: Students can add and subtract fluently through 10. Students understand that addition means “put together” and subtraction means to “take apart”.  Go to K.OA.A.5 to see previous skills in this progression.