Find sound frequency for ambulance?

Question

Can you walk me through both parts of this problem?

An ambulance with its siren (1000 Hz) turned on is traveling at 20 m/s. Assuming the speed of sound in air is 330 m/s, find the apparent frequency for an observer if

a) the ambulance is approaching the observer

b) the ambulance is moving away from the observer

Answer 1

n' (apparent) = n(real) [v - vo]/[v-vs]

v=330, vo = observer, vs = source

all Must be kept moving in the same direction > if one is not > put a -ve sign and resolve along

a) vo=0 (at origin), v=+330 (+x), vs = 20 (+x)

n' = 1000[330 -0]/[330 - 20] = 1064.5 Hz

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source has crossed the observer

b) vo=0 (at origin), v= - 330 (- x, sound reaching origin), vs = 20 (+x)

n' = 1000[-330 -0]/[-330 - 20] = 942.86 Hz