\[\boxed{\text{577}\text{\ (577)}\text{.}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ c_{1} = 20;\ \ d = 3:\]
\[c_{n} = 20 + 3 \cdot (n - 1);\ \]
\[c_{5} = 20 + 3 \cdot 4 = 20 + 12 = 32.\]
\[\textbf{б)}\ c_{1} = 5,8;\ \ \ d = - 1,5:\]
\[c_{n} = 5,8 - 1,5 \cdot (n - 1);\]
\[c_{21} = 5,8 - 1,5 \cdot 20 = - 24,2.\ \]