A helicopter takes off along the vertical with an acceleration $a=3 \mathrm{m} {\mathrm{s}}^{-2}$ and zero initial velocity. In a certain time $t_1$, the pilot switches off the engine. At the point of take-off, the sound dies away in time $t_2=30 \mathrm{s}$. Determine the velocity $v$ of the helicopter (in $\mathrm{m} {\mathrm{s}}^{-1}$) at the moment when its engine is switched off, assuming that the velocity $c$ of sound is $320 \mathrm{m} {\mathrm{s}}^{-1}$.

Hence, the correct answer is 80.