Which practice aligns with adding numbers within 20 by decomposition?

• A. Encouraging students to decompose one of the numbers so it can make a ten with the other number.
• B. Having students memorize all sums up to 20.
• C. Relying on counting on fingers only.
• D. Using subtraction to check answers.

Answer: (A)

Decomposition to form a ten with the other addend is a powerful way to simplify adding numbers within 20. The idea is to break one number into parts so that part of it pairs with the other number to make 10, then add the remaining part. For example, with 8 + 7, you can break 7 into 2 and 5, so 8 + 2 makes 10, and then you add the leftover 5 to get 15. This uses a familiar, easy-to-work-with benchmark (the number 10) to reduce cognitive effort and speed up mental math, while also building flexible number sense for future problems.

Other approaches don’t use this ten-based decomposition. Memorizing all sums up to 20 doesn’t develop the strategy itself, counting on fingers focuses on a different counting method, and using subtraction to check answers doesn’t teach the additive decomposition technique.