Phone plans A and B each have a different flat monthly rate for the first 500 text messages and a different constant rate for every additional message over 500

Question

Plan A charges $50 monthly for 500 text messages and $66 monthly for 580 text messages.

Plan B charges $45 monthly for 500 text messages and $69 monthly for 580 text messages.

 Write equations for Plan A and Plan B. Let x represent the number of messages over 500 and y represent the total monthly charges. 

Answer 1

Plan A: (66-50)/(580-500) = 16/80 = 1/5 = 0.20 per text message over 500

y = 0.2X + 50 if x>500

Plan B: (69-50)/(580-500) = 19/80 = 0.2375 per text message over 500

y = 0.2375 X + 50 if x>500