A welding operation is being performed with voltage $= 30 \: V$ and current =$100 \: A$. The cross-sectional area of the weld bead is $20 \: mm^2$. The work-piece and filler are titanium for which the specific energy of melting is $14 \: J/mm^3$. Assuming a thermal efficiency of the welding process $70 \%$, the welding speed (in $mm/s$) is __________ (correct to two decimal places).