\[b_{n} = 2n + 1\]
\[b_{1} = 2 + 1 = 3;\]
\[b_{2} = 4 + 1 = 5;\]
\[d = 5 - 3 = 2.\]
\[S_{20} = \frac{2a_{1} + 19d}{2} \cdot 20 =\]
\[= 10 \cdot (2 \cdot 3 + 19 \cdot 2) = 10 \cdot 44 = 440.\]
\[Ответ:440.\]