Doppler ultrasound is used to measure the speed of blood flow. The reflected sound interferes with the emitted sound, producing beats. If the speed of red blood cells is $0\mathrm{.}100$ m$/$s$,$ the ultrasound frequency used is $6\mathrm{.}70$ MHz$,$ and the speed of sound in blood is $1570$ m$/$s$,$ what is the beat frequency? If the speed of the red blood cells is v$,$ the speed of sound in blood is u$,$ the ultrasound source emits waves of frequency f$,$ and we assume that the blood cells are moving directly toward the ultrasound source, then the frequency fr of reflected waves detected by the apparatus is given by fr $=$ F$\frac{u+v}{u-v}.$ Express your answer in H$z$