Harry's Rent-A-Heap will rent you a rusted-out, ten-year old Ford van for $25 per day plus $.50 a mile.
How much will you pay if you rent the van for 3 days and drive it 245 miles?
How much will you pay if you rent it for days and drive it miles? (The answer is a formula, of course.)
Solution
And there should be part c: what if the van breaks down while you're driving it? Who pays? Was that made clear before you signed the deal?