9000033708

A stone has been thrown vertically up at the velocity $15\mathrm{m}{\mathrm{s}}^{-1}$ from the initial height $10\mathrm{m}$. How long (in seconds) has been the height of the stone at least $20\mathrm{m}$? Hint: The height $h$ is given by the expression $h=s_0+v_{0}t-\frac{1}{2}g{t}^2$, the standard acceleration is $g\doteq 10\mathrm{m}{\mathrm{s}}^{-2}$.