\[a_{1} = 1;\ \ a_{2} = 6:\]
\[d = a_{2} - a_{1} = 6 - 1 = 5.\]
\[S_{20} = \frac{2a_{1} + 19d}{2} \cdot 20 =\]
\[= 10 \cdot (2 + 19 \cdot 5) = 10 \cdot 97 = 970.\]
\[Ответ:970.\]