Here we can use general formula of Doppler effect.

${\mathrm{f}}_{\mathrm{app}}={\mathrm{f}}_{\mathrm{n}}\left(\frac{\mathrm{v}-{\mathrm{v}}_{\mathrm{o}}}{\mathrm{v}-{\mathrm{v}}_{\mathrm{s}}}\right)$   ………(i)

The direction of sound wave is always from source to observer. In this case both the source and observer are moving towards each other. The direction of motion of observer is opposite to the velocity of sound. So Eq. (i) becomes

${\mathrm{f}}_{\mathrm{app}}={\mathrm{f}}_{\mathrm{n}}\left(\frac{\mathrm{v}-\left(-{\mathrm{v}}_{\mathrm{o}}\right)}{\mathrm{v}-{\mathrm{v}}_{\mathrm{s}}}\right)={\mathrm{f}}_{\mathrm{n}}\left(\frac{\mathrm{v}+{\mathrm{v}}_{\mathrm{o}}}{\mathrm{v}-{\mathrm{v}}_{\mathrm{s}}}\right)$

The speed of source and observer is equal to half the speed of sound.

Hence ${\mathrm{f}}_{\mathrm{app}}={\mathrm{f}}_{\mathrm{n}}\left(\frac{\mathrm{v}+\frac{\mathrm{v}}{2}}{\mathrm{v}-\frac{\mathrm{v}}{2}}\right)=3{\mathrm{f}}_{\mathrm{n}}$

Percentage change in frequency

$\mathrm{\%}\text{ change }=\frac{{\mathrm{f}}_{\text{app }}-{\mathrm{f}}_{\mathrm{n}}}{{\mathrm{f}}_{\mathrm{n}}}\times 100=\frac{3{\mathrm{f}}_{\mathrm{n}}-{\mathrm{f}}_{\mathrm{n}}}{{\mathrm{f}}_{\mathrm{n}}}\times 100=200\mathrm{\%}$