A car $P$ travelling at $20 \mathrm{~ms}^{-1}$ sounds its horn at a frequency of 400 Hz . Another car $Q$ is travelling behind the first car in the same direction with a velocity $40 \mathrm{~ms}^{-1}$. The frequency heard by the passenger of the car Q is approximately [Take, velocity of sound $=360 \mathrm{~ms}^{-1}$ ]
