There is a fixed cost and a variable cost (depends on the number of miles driven).
Note that the fixed cost is $180.
The variable cost is ($0.15/mile)*(number of miles). [miles cancel out, leaving only $]
Total cost = fixed cost + variable cost
Total cost = $180 + $0.15x [where x is the number of miles driven]
So, if the total cost is $285,
$285 = $180 + $0.15x
$105 = $0.15x [subtract $180 from both sides]
700 = x [divide both sides by $0.15]
You can drive 700 miles.