If x represents the number of cars that wash Aron then the domain of x must contain only the whole and positive numbers (natural numbers)
This, because Aron can not wash -4 cars, it would be absurd then that x was a negative number.
In the same way Aron can not wash 1.57 cars. Or washed 1 car or washed 2, it is not consistent in this case to say that washed a car and a half, for example.