A car rental company's standard charge includes an initial fee plus an additional fee for each mile driven. The standard charge S (in dollars) is given by the function S = 0.40M +14.95, where M is the number of miles driven.

The company also offers an option to insure the care against damage. The insurance charge I (in dollars) is given by the function I=0.15M+3.90.

Let C be the total charge (in dollars) for a rental that includes insurance. Write an equation relating C to M. Simplify your answer as much as possible.

C=