The driver of a car travelling with speed 30 ~ms^ -1 towards a hill...

Key Idea Apply Doppler's effect. Whenever there is a relative motion between a source of sound and the observer (listner), the frequency of sound heard by the observer is different from the actual frequency of sound emitted by source.

[for case I] $\quad \mathbf{n}^{\prime}=\frac{\mathrm{v}}{\mathrm{v}-30} \mathbf{n}$ [for case II] $n^{\prime \prime}=\frac{v+30}{v} n^{\prime}$ From Eqs. (i) and (ii), we get $$\begin{aligned} \mathrm{n}^{\prime \prime}=\frac{\mathrm{v}+30}{\mathrm{v}-30} \mathrm{n}& =\frac{360}{300} \times600 \\ & =720 \mathrm{ Hz} \end{aligned}$$