A car $P$ travelling at $20 \mathrm {~m} \mathrm {~s} ^ { - 1 }$ 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 $40 \mathrm {~m} \mathrm {~s} ^ { - 1 }$. The frequency heard by the passenger of the car $Q$ is approximately [Take, velocity of sound $= 360 \mathrm {~m} \mathrm {~s} ^ { - 1 }$]
(1) 421 Hz
(2) 471 Hz
(3) 485 Hz
(4) 514 Hz