Final answer:
The car rental agency should charge $32 per day to maximize income.
Step-by-step explanation:
To maximize income, the car rental agency should charge a rental cost that allows all 42 cars to be rented. According to the information given, at a price of $32 per day, all the cars can be rented. However, for each $1 increase in rental cost, one car is not rented. This means that if the agency charges $33 per day, one car will not be rented, resulting in a loss of income. Therefore, the agency should continue charging $32 per day to maximize its income.