For his services, a private investigator requires a 500 dollar retainer fee plus 80 dollar per hour. Let $x$ represent the number of hours the investigator spends working on a case.
a. Find a function $f$ that models the investigator's fee as a function of $x$.
b. Find $f^{-1} .$ What does $f^{-1}$ represent?
c. Find $f^{-1}(1220) .$ What does your answer represent?