The velocity of sound in a gas at a fixed temperature is given by
$$
v=\sqrt{\frac{\gamma R T}{M}}
$$
where, $\gamma=\left(\frac{C_{p}}{C_{V}}\right)$ is specific heat ratio and $M$ is the molecular mass of the gas.
Let, velocity of sound in hydrogen, $v_{1}=\sqrt{\frac{\gamma R T}{M_{1}}}$
Velocity of sound in oxygen, $v_{2}=\sqrt{\frac{\gamma R T}{M_{2}}}$
Let, $M_{1}$ and $M_{2}$ be the molecular mass of hydrogen and oxygen respectively $n_{1}$ and $n_{2}$ be the moles of hydrogen and oxygen, respectively.
Molecular mass of the mixture,
$$
M_{\text {mix }}=\frac{n_{1} M_{1}+n_{2} M_{2}}{n_{1}+n_{2}}
$$
According to the question, $n_{1}=n_{2}$ at given NTP.
(given)
$$
\rightarrow \quad \ddot{u}_{\text {aix }}=\sqrt{\frac{2 \gamma R T}{17 M_{1}}}
$$
$\therefore$ The ratio of velocity of sound in mixture to that of hydrogen
$$
\begin{aligned}
\frac{v_{1}}{v_{\operatorname{mix}}} &=\frac{\sqrt{\frac{\gamma R T}{M_{1}}}}{\sqrt{\frac{\gamma 2 R T}{17 M_{1}}}} \\
\Rightarrow \quad \frac{v_{\text {mix }}}{v_{1}} &=\sqrt{\frac{2}{17}}
\end{aligned}
$$