The frequency of a radar is 780 MHz. The frequency of the reflected wave from an aeroplane is increased by 2 .6 kHz. The velocity of aeroplane is

See questiotr (08). 
$$\begin{gathered} n^{\prime }-n^{\prime \prime }=\Delta n=\frac{2V_s}{V}\times n \\ \Delta n=2\cdot 6kHz=2\cdot 6\times {10}^{3}Hz \\ \text{ and }n=780MHz=780\times {10}^{6}Hz \\ \therefore 2\cdot 6\times {10}^3=\frac{2V_S}{3\times {10}^8}\times 780\times {10}^6 \\ \text{ or }{v}_s=\frac{\left(2\cdot 6\times {10}^3\right)\left(3\times {10}^8\right)}{2\times \left(780\times {10}^6\right)}=500m/s \\ =0\cdot 5km/sec \end{gathered}$$