Under standard conditions, the density of a gas is 1.3 mg cm ^ - 3...

${V}_{sound} = \sqrt{\dfrac{\gammaP}{ρ}}$ and $\gamma = 1 + \dfrac{2}{f}$

$∴$ $1 + \dfrac{2}{f} = \dfrac{{v}_{sound}^{2} \times ρ}{P}$

$1 + \dfrac{2}{f} = \dfrac{( 330{)}^{2} \times ( 1 . 3 )}{1 . 01 \times {10}^{5}}$

$1 + \dfrac{2}{f} = 1 . 40$

$f = 5$