Two source P and Q produce notes of frequency 660 ~Hz each. A...

\text{ Accordingto the question }<img src="/api/proxy-image?url=https%3A%2F%2Ffirebasestorage.googleapis.com%2Fv0%2Fb%2Fquantrexacademy-app.firebasestorage.app%2Fo%2Fquestions%252Ffigs%252Fpyq%252Fneet%252FPta9DMV6RYmZtCumA88GYIvIEaXYZsRsGFGsA9ZpV60.original.fullsize.png%3Falt%3Dmedia&clean=1&v=wm2" data-qx-orig-src="https://firebasestorage.googleapis.com/v0/b/quantrexacademy-app.firebasestorage.app/o/questions%2Ffigs%2Fpyq%2Fneet%2FPta9DMV6RYmZtCumA88GYIvIEaXYZsRsGFGsA9ZpV60.original.fullsize.png?alt=media" data-qx-storage-src="https://firebasestorage.googleapis.com/v0/b/quantrexacademy-app.firebasestorage.app/o/questions%2Ffigs%2Fpyq%2Fneet%2FPta9DMV6RYmZtCumA88GYIvIEaXYZsRsGFGsA9ZpV60.original.fullsize.png?alt=media"> \\ Speed of listenerV_L=1 \mathrm{~ms}^{-1}$Speed of sound$V=330 \mathrm{~ms}^{-1}$Frequency of each source$v=600 \mathrm{~Hz}\therefore$Apparent frequency due to$\mathrm{P}=\mathrm{V}^{\prime}=\frac{v\left(\mathrm{~V}-\mathrm{V}_{\mathrm{L}}\right)}{\mathrm{V}} Apparent frequency due to $\mathrm{Q}=\mathrm{V}^{\prime \prime}=\frac{v\left(\mathrm{~V}-\mathrm{V}_{\mathrm{L}}\right)}{\mathrm{V}}\therefore$ No. of heat heard by the listener per second.$$\begin{aligned}& \mathrm{V}^{\prime \prime}-\mathrm{V}^{\prime}=\frac{v\left(\mathrm{~V}-\mathrm{V}_{\mathrm{L}}\right)}{\mathrm{V}}-\frac{v\left(\mathrm{~V}_0-\mathrm{V}_{\mathrm{L}}\right)}{\mathrm{V}}\\ & =\frac{2 v_{\mathrm{L}}}{v}=\frac{2 \times 660 \times 1}{330}=4 \end{aligned}$$