What is the measure of an exterior angle of a regular quadrilateral

- - Any "regular" polygon has all sides equal AND all angles equal.

- - The sum of the angles inside a 4-sided figure is 360°.

- - So it it's a regular figure, then each angle is 360°/4 = 90°.

- - The total angle around each vertex is 360°.

If 90° of them are inside, then the exterior angle is the remaining 270°.