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## Homework Statement

The pitch of the sound from a racecar engine drops the musical interval of a fourth when it passes the spectators. This means the frequency of the sound after passing is 0.75 times what it was before. How fast is the racecar moving?

1. 61 m/s

2. 49 m/s

3. 72 m/s

4. 68 m/s

## Homework Equations

fo = fs / 1 - (vs/v)

## The Attempt at a Solution

I tried using 75 = 100 / (1 - (vs/331) to solve for the speed of the source, but my answer was way off...there's got to be something i'm missing. am i correct in thinking i need to use v = 331 for the speed of sound? then what do i use for fo and fs?