\[\boxed{\text{611}\text{\ (611)}\text{.}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[x_{1} = 21,\ \ d = - 0,5:\ \text{\ \ \ \ \ \ \ \ \ \ }\]
\[x_{6} = 21 - 0,5 \cdot (6 - 1) = 18,5;\]
\[x_{25} = 21 - 0,5 \cdot (25 - 1) = 21 - 12 = 9;\ \]
\[S_{n} = \frac{\left( x_{1} + x_{n} \right)}{2} \cdot n;\]
\[S_{20} = \frac{18,5 + 9}{2} \cdot 20 = 27,5 \cdot 10 = 275.\]