For this case we have to:
d: Let the variable representing the number of miles traveled per day
Then, the cost will be given by the following equation:
[tex]C (d) = 0.85d + 40[/tex]
If we have the cost [tex]C (d) = 91[/tex], then the number of miles traveled is:
[tex]91 = 0.85d + 40\\91-40 = 0.85d\\51 = 0.85d\\d = \frac {51} {0.85}\\d = 60[/tex]
Thus, the number of miles traveled is 60.
Answer:
[tex]C (d) = 0.85d + 40[/tex]
For a [tex]C (d) = 91[/tex], the number of miles traveled is 60
