\[\boxed{\text{585\ (}\text{с}\text{).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[y_{n} = y_{1} + d(n - 1)\text{\ \ }\]
\[y_{n} - y_{1} = d(n - 1)\]
\[d = \frac{y_{n} - y_{1}}{n - 1}\]
\[\textbf{а)}\ y_{1} = 10;\ \ y_{5} = 22:\ \]
\[d = \frac{22 - 10}{4} = \frac{12}{4} = 3.\]
\[\textbf{б)}\ y_{1} = 28;\ \ y_{15} = - 21:\]
\[d = \frac{- 21 - 28}{14} = - 3,5.\]