3 Two mobile phone companies have different tariffs for text messages: Phone company X charges $20 per month for unlimited texts. .
Phone company Y charges $5 per month and $0.05 per text. .
a) For each company, write a formula that gives the total monthly charge
($C) for someone who sends n texts.
b) Draw a graph of C against n for both companies on the same axes,
showing the monthly charges for up to 500 texts.
c) From your graph estimate the number of texts at which both companies
would have the same monthly charge.