Two cars X and Y are approaching each other with velocities 36 km / h and 72 km / h respectively. The frequency of a whistle sound as emitted by a…

Two cars $\displaystyle X$ and $\displaystyle Y$ are approaching each other with velocities $\displaystyle 36 \mathrm{~km} / \mathrm{h}$ and $\displaystyle 72 \mathrm{~km} / \mathrm{h}$ respectively. The frequency of a whistle sound as emitted by a passenger in car X, heard by the passenger in car Y is $\displaystyle 1320$ Hz . If the velocity of sound in air is $\displaystyle 340$ m/s, the actual frequency of the whistle sound produced is \_\_\_\_ Hz.