A submarine $\mathrm{A}$ travelling at $18\mathrm{km} {\mathrm{hr}}^{-1}$ is being chased along the line of its velocity by another submarine $\mathrm{B}$ travelling at $27\mathrm{ }\mathrm{km} {\mathrm{hr}}^{-1}$ . $\mathrm{B}$ sends a sonar signal of $500\mathrm{ }\mathrm{H}\mathrm{z}$ to detect $\mathrm{A}$ and receives a reflected sound of frequency $\mathrm{v}$. The value of $\mathrm{v}$ is closed to (Speed of sound in water $1500 \mathrm{m }{\mathrm{s}}^{-1}$ )