A car traveling 35.0 m/s overtakes another car going only 22.0 m/s. When the faster car is still behind the slower one, it sounds a horn of frequency 1500.0Hz. What is the frequency heard by the driver of the slower car?

Answer 1

For observer and source both moving: fobserver=fsource [(1+ or - vobserver/v)/(1+ or -vsource/v)] v= velocity of sound in air signs in numerator: + if observer moving toward source and - if away from source signs in denominator: - if source moving toward observer and + if away from observer

Reference https://www.physicsforums.com/threads/doppler-effect-source-observer-both-moving.625483/