A linear pair of angles is formed when two lines intersect. That's why it's said that these angles are supplementary because they sum 180 degrees.
Statement Reason.
1. Angle 1 and 2 are a linear pair. Given.
2. Angle 2 = 115. Given.
3. Angle 1 + Angle 2 = 180. Definition of linear pairs.
4. Angle 1 + 115 = 180. By statement 2.
5. Angle 1 = 180-115 = 65 Subtracting on both sides by 115.
Therefore, angle 1 = 65.