If it costs $32 a day, and you know that you will have the car 5 days, then you can figure that out first with simply:

5(days) x 32(dollars per day) = $160

Now we see that we have $90 left in our budget. If you have $90 and it costs $0.15 per mile driven and you want M miles, you can set up the equation:

(0.15)M <= 90, where M is miles driven, and 90 is the amount of budget you have left.

If you solve this, you can see that the maximum miles you can drive is 600 miles without going over your budget.