$18)$Each piston of an engine makes a sharp sound every other revolution of the engine.

$($a$)$ How fast $($in $k\frac{m}{h}r)$ is a race car going if its eight$-$cylinder engine emits a sound of frequency $663Hz,$ given that the

engine makes $2472$ revolutions per kilometer?

$k\frac{m}{h}r$

$($b$)$ At how many revolutions per minute is the engine rotating?

$re\frac{v}{m}in$