If your price is $10, you rent all 24 cars; yielding: 10*24 = $240.
If your price is $11, you rent only 23 cars; yielding: 11*23 = $253.
If your price is $12, you're down to 22 cars; yielding: 12*22 = $264.
You see the pattern? As you can see, the profit is rising.
However, you can't keep doing this since the number of cars will go to zero.
In general, if the price is (10+n), you'll be renting (24-n) cars.
The income I(n) is still the product of the price with the number of cars.