We are given equation: c = 12w + 20, that represents total cost for w number of weeks.
We need to describle, what happen when w that is number of week increases.
w is an independent variable there. But c is a dependent variable.
As the number of weeks w increase the total cost c increases. But that would not effect fix $20 sign-up fee for joining a video streaming service.
Therefore, we could say:
The number of weeks and the total cost increase, but the sign-up fee stays the same.