\[\boxed{\text{991\ (991).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[a_{3} = 150,\ \ a_{13} = 110,\ \ \]
\[S_{n} = 0,\]
\[a_{3} = a_{1} + 2d = 150,\]
\[a_{13} = a_{1} + 12d = 110.\]
\[\left\{ \begin{matrix} a_{1} + 12d = 110\ \ \\ a_{1} + 2d = 150\ \ \ \ \\ \end{matrix} \right.\ ( - ) \Longrightarrow\]
\[S_{n} = \frac{2a_{1} + d(n - 1)}{2} \cdot n,\]
\[\frac{2a_{1} + d(n - 1)}{2} \cdot n = 0\]
\[\left( 2 \cdot 158 - 4 \cdot (n - 1) \right) \cdot n = 0\]
\[316n - 4n^{2} + 4n = 0,\ \ n > 0,\]
\[n² - 80n = 0\]
\[n(n - 80) = 0\]
\[n = 80.\]
\[Ответ:сложили\ 80\ членов\ \]
\[прогрессии,\ начиная\ с\ первого.\]