Ответ:
\[S = - 30t + 5t^{2} =\]
\[= 5 \cdot \left( t^{2} - 6t + 9 \right) - 45 =\]
\[= 5 \cdot (t - 3)^{2} - 45.\]

\[Ответ:t = 3\ с.\]
\[S = - 30t + 5t^{2} =\]
\[= 5 \cdot \left( t^{2} - 6t + 9 \right) - 45 =\]
\[= 5 \cdot (t - 3)^{2} - 45.\]

\[Ответ:t = 3\ с.\]