# 4. Relate Addition and Subtraction

Understand that addition and subtraction are reverse operations, and visualize subtraction as an unknown addend problem. For example, 9 - 7 = ? can be viewed as 7 + ? = 9. Therefore, knowledge of addition facts can be used to solve subtraction problems. Students also construct and complete related addition and subtraction facts involving 3 numbers (fact families).

### Common Core Alignment

**1.OA.4**Understand subtraction as an unknown-addend problem.