Frequency of the horn is given as $\mathrm{v}_{\mathrm{H}}=200 \mathrm{~Hz}$

Velocity of the man is given as $\mathrm{v}_{\mathrm{T}}=20 \mathrm{~m} / \mathrm{s}$

Velocity of sound is given as $v=340 \mathrm{~m} / \mathrm{s}$

(a) We know,

(i) The apparent frequency of the horn as the man approaches the observer is:

$v^{\prime}=v_{H}\left[v /\left(v-v_{T}\right)\right]$

$=200[340 /(340-20)]$

$=212.5 \mathrm{~Hz}$

(ii) The apparent frequency of the horn as the man runs away from the observer is:

$v^{\prime \prime}=v_{\mathrm{H}}\left[v /\left(v+v_{T}\right)\right]$

$=200[340 /(340+20)]$

$=188.88 \mathrm{~Hz}$

(b) In all circumstances, the speed of sound is $340 ms^{-1}$. The relative motions of the observer and the source cause the apparent shift in frequency.