To find the measure of an exterior angle of a polygon, you can use the formula:
\[ \text{Exterior Angle} = \frac{360°}{n} \]
where \( n \) is the number of sides of the polygon. In this case, Rhianna is working with a polygon that has five sides (a pentagon).
Substituting \( n = 5 \):
\[ \text{Exterior Angle} = \frac{360°}{5} = 72° \]
Therefore, the angle measure of any one exterior angle is 72°.