A parking garage has two entrances. Let X be the number of cars that enter the garage through door A in an hour, and Y be the number of cars that enter through door B in an hour. Assuming that _x= 15 and _y= 25, what is the mean of Z, the total number of cars that enter the garage in an hour.

a. 10

b. 15

c. 25

d. 40

e. The mean of Z cannot be determined.