An idiot following a fire truck northbound up the side of the highway is going 70 mph while the fire truck is going 60 mph. The siren on the fire truck is emitted with a frequency of 400 Hz. A moron on the southbound side of the highway slows to 20 mph to watch the fire truck.
a) What is the frequency heard by the idiot?
b) What is the frequency heard by the moron?