Geometry

Question Analysis:
This question assesses understanding of exterior angles in quadrilaterals, emphasizing their sum.

Key Concept Explanation:
The sum of all exterior angles of any polygon is 360°.
Quadrilaterals have four exterior angles (one per vertex).

Step-by-Step Solution:
Sum of given exterior angles: 80° + 90° + 100° = 270°.
Fourth exterior angle = 360° - 270° = 90°.

Common Mistakes:
Forgetting the 360° rule for exterior angles.
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