The source is emitting sound of frequency 'n'. A source and a...

The apparent frequency heard by the listener is given by the Doppler effect formula:

$n' = n \left( \dfrac{V + v_L}{V - v_S} \right)<br><br>Since both$ the $source and the listener are moving towards each other with speed \dfrac{V}{3}:<br><br>$ v_L = $\dfrac{V}{3}<br><br>$ v_S = $\dfrac{V}{3}<br><br>Substituting these values into$ the formula:

n' = n \left( \dfrac{V + \dfrac{V}{3}}{V - \dfrac{V}{3}} \right)$<br><br>$n' = n \left( \dfrac{\dfrac{4V}{3}}{\dfrac{2V}{3}} \right)

$n' = 2n$

Answer: 2n