Addition is the first of 4 basic operations in arithmetic. For most students it is also the easiest. Here is a list of resources that I use.

Addition Whole Numbers/No Carry Over/Units
Addition Whole Numbers/No Carry Over/ Tens
Addition Whole Numbers/No Carry Over/ Hundreds
Addition Whole Numbers/No Carry Over/ Thousands

Exercises/Practice sheets such as this go a long way in reinforcing this skill

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