Your equation is like that because you always pay $20 per month, no matter how many minutes you used. Then you have 0.15 per minute so use x as a variable placeholder for the actual number of minutes.
domain is starting from 0 because you can't have negative minutes.
range is at least 20 because you pay $20 to use it even if you don't use any minutes. So it can't be less than $20.
