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.