Questions: Find the measure of an exterior angle of a regular polygon with 18 sides.

To find the measure of an exterior angle of a regular polygon, you can use the formula:

\[ \text{Exterior angle} = \frac{360^\circ}{n} \]

where \( n \) is the number of sides of the polygon.

For a regular polygon with 18 sides:

\[ \text{Exterior angle} = \frac{360^\circ}{18} = 20^\circ \]

Therefore, the measure of an exterior angle of a regular polygon with 18 sides is \( 20^\circ \).

In summary, the answer is \( 20^\circ \).