A blood flow meter uses the Doppler effect to measure the speed of blood. Suppose a sound wave at 80,000 Hz is emitted into a vein where the speed of sound is about 1500 m/s. The wave is Doppler-shifted as it is absorbed by the red blood cells moving toward a stationary receiver. The wave is then Doppler shifted again as it is reflected back toward the detector. Hence, there are two Doppler shifts, one as the red blood cells act as the “observer” and another as they act as a new source of sound. If the frequency of the returning sound is 80,020 Hz, what is the speed of the blood flow?