EAMCET Engineering 2006 Solved Paper

An observer is standing 500m away from a vertical hill. Starting between the observer and the hill, a police van sounding a siren of frequency 1000‌Hz moves towards the hill with a uniform speed. If the frequency of the sound heard directly from the siren is 970‌Hz, the frequency of the sound heard after reflection from the hill (in Hz ) is about, (velocity of sound =330ms−1 ) :