The function which represents the phone rate is a straight line, which can be gotten by the following formula:
[tex]y=mt+b[/tex]
Where m is the slope, and b the point where the y-axis is crossed by the line.
Let's get the slope first:
[tex]m=\frac{27-25}{20-0}=\frac{2}{20}=\frac{1}{10}[/tex]
And b = 25 because (0, 25) is where the line has crossed the cost axis.
Therefore:
[tex]y=C(t)=\frac{1}{10}t+25[/tex]
