The interior angles of a polygon add up to 1980°. How many diagonals does it have?

Explanation

1. 01

   $(n - 2) \times 180 = 1980$, so $n - 2 = 11$ and $n = 13$.

2. 02

   Diagonals = $\frac{13 \times (13 - 3)}{2} = \frac{130}{2} = 65$.