A basic cellular package costs $40 /month for 60 minutes of calling with an additional charge of $0.20 /minute beyond that time. The cost function C(x) for using x minutes would be

If you used 60 minutes or less, i.e. if if x≤60 , then C(x)=40 (the base charge).
If you used more than 60 minutes, i.e. (x−60) minutes more than the plan came with, you would pay an additional $0.20 for each of those (x−60) minutes. Your total bill would be C(x)=40+0.20(x−60) .


If you want to keep your bill at $50 or lower for the month, what is the maximum number of calling minutes you can use?