\(\frac{1}{2} \rho_{\text {water }} V^2=\rho_{\text {mercury }} g h\)
\(V=\sqrt{2 \times \frac{\rho_{\text {mercury }}}{\rho_{\text {water }}} \times g \times h}\)
\(=\sqrt{2 \times 13.6 \times 9.8 \times \frac{40}{100}}\)
\(\Rightarrow V=10.32 \,m / s\)