A2. In a certain polygon, the sum of the interior angles is $4140^{\circ}$. What is the number of diagonals of this polygon?
(A) 250
(B) 275
(C) 205
(D) 725
(E) 257
A2. We use the formula for the sum of the interior angles of a polygon $S_{n}=(n-2) \cdot 180^{\circ}$ and calculate the number of sides (vertices) of the sought polygon. We get $n=25$. We also use the formula for the number of diagonals $d_{n}=\frac{n(n-3)}{2}$ and get $d_{25}=275$.