Beat freq is the difference between the two observedfrequencies. The sound heard from the stationary tuba is 75 Hz.From the moving tuba?

\(\displaystyle{f}_{{{o}{b}{s}}}={{f}_{{{s}{o}{u}{r}{c}{e}}}{\left(\frac{{v}}{{v}}-{v}_{{{s}{o}{u}{r}{c}{e}}}\right)}}={75}\cdot{\left(\frac{{343}}{{{343}-{10}}}\right)}={77.25}{H}{z}\)

So the beat freq was:77.25 - 75 = 2.25 Hz

\(\displaystyle{f}_{{{o}{b}{s}}}={{f}_{{{s}{o}{u}{r}{c}{e}}}{\left(\frac{{v}}{{v}}-{v}_{{{s}{o}{u}{r}{c}{e}}}\right)}}={75}\cdot{\left(\frac{{343}}{{{343}-{10}}}\right)}={77.25}{H}{z}\)

So the beat freq was:77.25 - 75 = 2.25 Hz