What is an angle if two adjacent angles add up to 100 degrees?

The angles BOC and AOD, formed by two intersecting straight lines AC and BD, are vertical. The vertical angles are equal, so Angle 2 = Angle 3.
The same can be obtained from other reasoning:
Angle 3 + Angle 1 = 180 (adjacent), so Angle 3 = 180 – Angle 1;

Angle 2 + Angle 1 = 180 (adjacent), so Angle 2 = 180 – Angle 1;

Therefore, Angle 2 = Angle 3.

If Angle 2 = Angle 3, and Angle 2 + Angle 3 = 100 degrees,
then Angle 2 = Angle 3 = 100/2 = 50 degrees.

By the property of adjacent corners, Angle 1 = 180 – Angle 2 = 180 – 50 = 130 degrees.
Likewise, Angle 1 = 180 – Angle 3 = 180 – 50 = 130 degrees.

Answer: The AOB angle is 130 degrees.