The total monthly charges, $C, for a mobile plan consist of a fixed amount of $ 120 and a variable amount which depends on the usage. For every minute used, $0.20 is charged.
(i) If the duration of usage is 120 minutes, calculate the total monthly charges for the mobile plan
(ii) If the total monthly charges for the mobile plan are $50, find the duration of usage.
(iii) Write down a formula connecting C and n, where n is the number of minutes of usage.