As given, the volumes of hydrogen and oxygen in a mixture is $41 $, so let $V$ be the volume of oxygen. The volume of hydrogen will be $4 \,V$.
If $\rho_{m}$ be the density of mixture, then
$\rho_{m}=\frac{4 V \times 1+V \times 16}{5 \,V}=4$
As $v \propto\left(\frac{1}{\rho}\right)^{1 / 2}$.
$\therefore$ Velocity in mixture $=\frac{1270}{(4)^{1 / 2}} $
$=635 \,m / s$