A car P travelling at $20 {\mathrm{ms}}^{-1}$sounds its horn at a frequency of $400 \mathrm{Hz}$. 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 $=360 {\mathrm{ms}}^{-1}$]