rent-4-wheels makes a profit of $12 per day for each car rented, and loses $4 per day for each car that is not rented. How many cars should the company have on hand to maximize profit, subject to the following probability distribution:

Question

rent-4-wheels makes a profit of $12 per day for each car rented, and loses $4 per day for each car that is not rented.  How many cars should the company have on hand to maximize profit, subject to the following probability distribution:
 
anticipated number of rental requests 
                 12    11      10    9      8      7      6
Probability   
                .09   .011   .22   .24   .12   .12   .10

This is what I did and  I;m not sure if it's correct. 
 
(12)(.09)+(4)(.91)=4.72
(11)(.11)+(4)(.89)=4.77
ect. i got that company should have ten cars to maximize profit