Example of a real life linear function?

Linear functions are used to model situations that show a constant rate of change between 2 variables. For example, the relation between feet and inches is always 12 inches/foot. so a linear function would be y = 12 x where y is the number of inches and x is the number of feet. y = 24 x models the number of hours in any given number of days {x}. Business applications abound. If a cell phone company charges a start-up fee of $50 and then $.05 for every minute used, the function is y = .05 x + 50.