$${v_\text{opt} = \sqrt{\mathrm{\frac{100\, kg \times 10 \, m/s^2}{1,2\, kg/m^3 \times \sqrt{0,1 \times 0,5\, \mathrm{m^2} \times 25\, \mathrm{m^2}}}}}=34\, \mathrm{\frac{m}{s}}}$$