A stationary source emits sound of frequency f _ 0 = 492Hz . The...

Frequency of sound as received by large car approaching the source.

${f}_{1} = \dfrac{C + {V}_{0}}{C}{f}_{0} = \left(\dfrac{330 + 2}{330}\right)492Hz .$

This car now acts as source for reflected sound wave

∴ {f}_{reflected} = {f}_{1}

Frequency of sound received by source,

${f}_{2} = \left(\dfrac{C}{C - {V}_{0}}\right){f}_{reflected} = \left(\dfrac{330}{330 - 2}\right) × {f}_{1} = \dfrac{330}{328} × \dfrac{332}{330} × 492Hz$

∴ Beat frequency $= \left|{f}_{0} - {f}_{2}\right| = \left(\dfrac{332}{328} - 1\right) × 492Hz$

$= 6Hz$