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## Homework Statement

"Each piston of an engine makes a sharp sound every other revolution of the engine. (a) How fast is a race car going if its eight-cylinder engine emits a sound of frequency 750 Hz, given that the engine makes 2000 revolutions per kilometer? (b) At how many revolutions per minute is the engine rotating?"

## Homework Equations

##f=750s^{-1}=750Hz=\frac{375}{\pi}\frac{rev}{s}##

##r=\frac{2000rev}{1km}##

## The Attempt at a Solution

##v=\frac{f}{r}=\frac{1km}{2000rev}(\frac{375}{\pi}\frac{rev}{s})=\frac{3}{16\pi}\frac{km}{s}=\frac{675}{\pi}\frac{km}{hr}##

The book says that it's ##v=340\frac{km}{hr}##. I don't understand what I am doing wrong.