Arental car costs $ 30 per day plus $ 0.20 per mile. if someone has $ 250 to spend and needs to drive the car 60 miles each day, for how many days can that person rent the car? assume that the car cannot be rented for part of a day.

So... for one day and no miles, the car costs 30*1, for 2 days 30*2, for 30*3 and so on, for "d" days, the car costs 30*d or 30d

now, if the driver drives it for 1 mile, the mileage costs is 0.20*1, for 2 miles is 0.20*2 for 3 miles is 0.20*3 and so on, for "m" miles, it costs 0.20*m or 0.2m

so, the actual costs to rent the car "d" days for "m" miles is really

C( d, m ) = 30d+0.20m

so.. if someone had only $250 to spend, and needs it for 60miles each day, how many days is that?  

solve for "d"