For this case, the first thing we must do is define variables. We have then: x: number of minutes y: total cost We have then the following description: The gym in a city charges $ 1 for every 20 minutes of training a person does every day. Write an equation that models the problem. The equation that models the problem is: Where the slope of the line is: And it represents the cost of the gym for every 20 minutes of training.