\[\boxed{\text{613}\text{\ (613)}\text{.}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[b_{1} = 4,2;\ \ \ b_{10} = b_{1} + 9d = 15,9:\ \ \ \ \]
\[4,2 + 9d = 15,9\]
\[9d = 11,7\]
\[d = 1,3\]
\[S_{15} = \frac{2c_{1} + d(n - 1)}{2} \cdot n = \frac{8,4 + 1,3 \cdot 14}{2} \cdot 15 = 199,5.\]