$${v = \frac{L_0 c}{\sqrt{(c \Delta t)^2 + L_0^2}} \\ = \frac{(230\, \mathrm{m})c}{\sqrt{(299\, 792\, 458\, \mathrm{m/s})^2(3,57 \times 10^{-6}\, \mathrm{s})^2 + (230\, \mathrm{m})^2}} \\ = \underline{\underline{0,210c}}}$$