Concept
Source is moving away from a stationary observer
$\therefore \mathrm{f}^{\prime}=\mathrm{f}\left(\frac{\mathrm{v}}{\mathrm{v}+\mathrm{v}_{\mathrm{s}}}\right)$
Where, $f$ is the true frequency $v$ is the speed of sound and $v_s$ is the speed of source.
Given, $\mathrm{f}=1152, \mathrm{v}=340 \mathrm{~m} / \mathrm{s}$
$\text { and } \mathrm{v}_{\mathrm{s}}=\frac{72 \times 10^3}{3600} \mathrm{~m} / \mathrm{s}=20 \mathrm{~m} / \mathrm{s}$
$\therefore$ the apparent frequency $\mathrm{f}^{\prime}=1152 \frac{\times(340)}{(340+20)} \mathrm{Hz}=1088 \mathrm{~Hz}$