According to the image below, what is the measure (in degrees) of angle ABC?

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90°

The exterior angles given at the upper triangle is on a straight line. Thus, it leads to the base and vertex angle. The vertex of the upper triangle and the middle bottom triangle are vertical angles. Thus, equals each other. Exterior angle D is on a straight line so, interior is solved. Then you can say opposite angles equals exterior angle or still use the sum of angles of a triangle to get your answer.

Any simpler form?

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