Problem $20$: A diagnostic ultrasound echo is reflected from moving blood and returns with a frequency $500$ Hz higher than its original $2\mathrm{.}00$ MHz $.$ The speed of the wave is $1540\frac{m}{s}.$ What is the velocity of the blood?$($Assume that the frequency of $2\mathrm{.}00$ MHz is accurate to seven significant figures and $500$ Hz is accurate to three significant figures.$)($Speed of sound is $(5$pt$)$