2011 AIME II Problem 3

The interior angles of an $18$-gon sum to $180 \cdot 16 = 2880$ degrees

With smallest angle $a$ and common difference $d,$ $2a + 17d = 320,$ so $d$ is even; convexity caps the largest angle below $180$

The interior angles of an $18$-gon sum to $180 \cdot 16 = 2880$ degrees. If the smallest angle is $a$ and the common difference is $d,$ then $18a + 153d = 2880,$ i.e. $2a + 17d = 320.$ Since $a$ and $d$ are integers, $17d$ must be even, so $d$ is even, and $d \ge 2$ because the sequence is increasing.

Convexity requires the largest angle $a + 17d = \frac{320 + 17d}{2}$ to be less than $180,$ so $17d \lt 40$ and $d \le 2.$ Thus $d = 2$ and $a = \frac{320 - 34}{2} = 143.$