Two engines pass each other moving in opposite directions with uniform speed of 30m/s. One of them is blowing a whistle of frequency 540Hz. Calculate the frequency heard by the driver of the second engine before they pass each other. Speed of sound is 330m/sec.

f=(c+vr)/(c-vs)fo
where
f:Observed frequency,
fo:source frequency
c:speed of sound
vr:velocity of the receiver relative to the medium (positive if the receiver is moving towards the source)
vs:velocity of the source relative to the medium (positive if the source is moving away from the receiver)
f=(330+30)/(330-30)×540=648 Hz