Let 't' denote the additional minutes beyond 60 minutes.
It is given that there is a monthly fixed cost of $19.95 for upto 60 minutes.
And then there is a variable cost of $0.39 per minute after that.
Then the total cost per month (C) is given by,
[tex]TotalCost=FixedCost+VariableCost\Rightarrow C(t)=19.95+0.39t[/tex]Thus, the equation for cost per month in terms of number of minutes of airtime, is obtained as,
[tex]C(t)=19.95+0.39t[/tex]