Answer:
40
Step-by-step explanation:
Angles ABO, BAO, and AOB are the angles of isosceles triangle AOB. The angles at A and B are equal, so we have ...
AOB +2ABO = 180° . . . . . sum of angles in the triangle
100° +2ABO = 180° . . . . . . use the given value
50° +ABO = 90° . . . . . . . . divide by 2; next, subtract 50°
∠ABO = 40°
