A parking garage attendant can wait on 40 cars per hour. If cars arrive randomly at a rate of $x$ cars per hour, then the average line length is given by
$$
f(x)=\frac{x^{2}}{1600-40 x^{\prime}}
$$
where the $x$ -values are limited to $0 \leq x<40$
(a) Solve the inequality $f(x) \leq 8$
(b) Interpret your answer from part (a).