the following problem is simplified with each step shown. Name the number property that justifies each step.

1. (13 + a) + 2 + 4a
2. (13 + a) + 4a + 2
3. 13 + (a + 4a) + 2
4. 13 + 5a + 2
5. 13 + 2 + 5a
6. 15 + 5a

Which number property would each of these steps be? Please help me. 60 points.