Answer
79.8 miles per hours fast is the car going.
Formula
By using the dopplers effect. (When source is approching .)
As given
A car approaches you with its horn blowing.
The observed frequency is 503.7 Hz.
The speed of sound is 1100 ft/s .
The car's horn has a frequency of 450 Hz.
i.e
v = 1100 ft/s
Put in the above formula
Now convert 117.3 ft per second into miles per hours.
117.3 feet per second = 79.8 miles per hours (Approx)
Thus
Therefore the 79.8 miles per hours fast is the car going.