An observer moves towards a stationary source of sound, with a velocity one-fifth of the velocity
of sound, What is the percentage increase in the apparent frequency ? 

$v_0=\frac{v}{5}\Rightarrow v_0=\frac{320}{5}=64{\mathrm{ms}}^{-1}$

When observer moves towards the stationary source, then

$n^{\mathrm{\prime }}=\left(\frac{v+v_0}{v}\right)n\Rightarrow n^{\mathrm{\prime }}=\left(\frac{320+64}{320}\right)n\text{ or }{n}^{\mathrm{\prime }}=\left(\frac{384}{320}\right)n\text{ or }\frac{n^{\mathrm{\prime }}}{n}=\frac{384}{320}$

The percentage increase in the apparent frequency $\left(\frac{n^{\mathrm{\prime }}}{n}-1\right)100\mathrm{\%}=\left(\frac{384}{320}-1\right)100\mathrm{\%}=20\mathrm{\%}$