A source of sound emitting a note of frequency n_0 is approaching a...

Using the Doppler effect formula for a source approaching a stationary listener:

$n' = n_0 \left( \dfrac{V}{V - V_s} \right)<br><br>Substituting$ the given values,n' = 3n_0$:<br><br>$3n_0 = n_0 \left( \dfrac{V}{V - V_s} \right)$<br><br>$3 = \dfrac{V}{V - V_s}$<br><br>$3V - 3V_s = V$<br><br>$2V = 3V_s$<br><br>$V_s = \dfrac{2V}{3}$<br><br>Answer:$\dfrac{2V}{3}