The speed of sound in hydrogen at STP is v. The speed of sound in a mixture containing 3 moles of hydrogen and 2 moles of oxygen at STP will be

Let the density of hydrogen be $\rho \text{'}=2\rho ,$ then the density of oxygen will be $32\rho$ . The density of the mixture will be
$\rho "=\frac{3}{5}\times 2\rho +\frac{2}{5}\times 32\rho =14\rho$
Since the pressure is the same, we have
$\frac{v"}{v}=\sqrt{\frac{\rho \text{'}}{\rho "}}=\sqrt{\frac{2\rho }{14\rho }}=\frac{1}{\sqrt{7}}$