Here, source is moving towards a stationary observer.

Given,

Velocity of source, $v_s=39.6 \,km / h$

$=39.6 \times \frac{5}{18}=11 \,ms ^{-1}$

Velocity of sound, $v=330 \,ms ^{-1}$

Frequency of source, $f=\frac{1}{30} \,s ^{-1}$

So, apparent frequency of sound as heard by the observer is

$f^{\prime}=\left(\frac{v}{v-v_s}\right) \cdot f=\left(\frac{330}{330-11}\right) \cdot \frac{1}{30}$

$\Rightarrow \quad f^{\prime}=\frac{11}{319}$

$\Rightarrow \quad f^{\prime}=\frac{1}{29}$

So, time interval between two honks of bus horn observed by observer (person at bus stop) is $29 \,s$.