When the observer in car $Q$ hears the sound of the horn, the effective velocity of approach of the car $\mathrm{P}$ towards observer is $\mathrm{v}_{\mathrm{s}} \cos \theta.$

Thus, apparent frequency heard by the observer in car $Q$ is

${v^{\prime}=\left(\frac{v}{v-v_{s} \cos \theta}\right) v} $

Here ${\cos \theta=\frac{4}{5}}$

$\therefore $ $ v^{\prime}=\frac{340}{340-10 \times \frac{4}{5}} \times 700=\frac{340}{332} \times 700 $

$ =716.86 \mathrm{Hz} $

$ \approx 717 \mathrm{Hz} $