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 = 15.75 + 0.50M , where M is the number of miles driven. The company also offers an option to insure the car against damage. The insurance charge
I (in dollars) is given by the function I = 3.90 + 0.15M. 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.

Question

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 = 15.75 + 0.50M , where M is the number of miles driven. The company also offers an option to insure the car against damage. The insurance charge 
I (in dollars) is given by the function I = 3.90 + 0.15M. 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.