Answer:
The measure of an interior angle of a regular 15-gon is 120°.
Explanation:
We need to determine the measure of the size of an interior angle of a regular 15-gon having 15 sides.
 Thus,
The number of sides n = 15
Hence, 
 Using the formula to determine the measure of an interior angle of a regular 15-gon is given by 
 (n - 2) × 180° = n × interior angle 
 substitute n = 15 
 (15 - 2) × 180 = 15 × interior angle
13 × 180 = 15 × interior angle
Interior angle = (10 × 180) / 15
 = 1800 / 15
 = 120°
Therefore, the measure of an interior angle of a regular 15-gon is 120°.