Geometry

Question Analysis:
This is an algebraic application of the interior angle formula for a regular polygon.
The main focus is setting an algebraic expression equal to the calculated interior angle.

Key Concept Explanation:
Each interior angle = [(n - 2) × 180] / n

Step-by-Step Solution:
For n = 10, interior angle = [(10 - 2) × 180] / 10 = 144°
We are told:
3x + 15 = 144
Subtract 15:
3x = 129
x = 43

Common Mistakes:
Using sum of interior angles...