A basic cellular package costs $30/month for 60 minutes of calling with an additional charge of $0.40/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)=30 (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 $0.40 for each of those (x−60) minutes. Your total bill would be C(x)=30+0.40(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?
 
The maximum calling minutes you can use is   minutes.