From Mechanics

\displaystyle \begin{align} & 2t-\frac{{{t}^{2}}}{10}=0 \\ & t\left( 2-\frac{t}{10} \right)=0 \\ & t=0\text{ or }t=20 \\ \end{align}

\displaystyle t=0 is not of physical interest thus the answer is \displaystyle t=20\ \text{s}.