Answer:
280°
Explanation:
You want the measure of external angle x in a quadrilateral with three external angles marked x and one marked 240°.
Angle sum
The sum of the internal angles of a quadrilateral is 360°. The internal angle at each vertex will be the difference between 360° and the external angle:
(360° -240°) +3(360° -x) = 360°
120° +3(360° -x) = 360°
3(360° -x) = 240°
360° -x = 80°
x = 360° -80° = 280°
The size of angle x is 280°.
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