Two cars X and Y are approaching each other with velocities 36km h...

Two cars $X$ and $Y$ are approaching each other with velocities $36km{h}^{- 1}$ and $72km{h}^{- 1}$ respectively. The frequency of a whistle sound as emitted by a passenger in car $X,$ heard by the passenger in car $Y$ is $1320Hz .$ If the velocity of sound in air is $340{ms}^{- 1},$ the actual frequency of the whistle sound produced is ________ $Hz .$