$\mathrm{v}^{\prime}=\mathrm{v}_{0}\left(\frac{\mathrm{v}+\mathrm{v}_{0}}{\mathrm{v}}\right)$

where

$\mathrm{v}_{0}=$ frequency fo the source

$\mathrm{v}_{0}=$ velocity of the observer

$\mathrm{v}=$ velocity of the sound

$\therefore $ $\mathrm{v}^{\prime}=\mathrm{v}_{0}\left(\frac{\mathrm{v}+\frac{\mathrm{v}}{5}}{\mathrm{v}}\right)=\frac{6 \mathrm{v}_{0}}{5}$

Percentage change in apparent frequency

$=\frac{v^{\prime}-v_{0}}{v_{0}} \times 100$

$=\frac{\left(\frac{6 v_{0}}{5}-v_{0}\right)}{v_{0}} \times 100=\frac{1}{5} \times 100=20 \%$