There are five levels defined along the continuum of the CMM and, according to the SEI: "Predictability, effectiveness, and control of an organization's software processes are believed to improve as the organization moves up these five levels. While not rigorous, the empirical evidence to date supports this belief."
As I look at amountOf, I can see that it uses information from the rental, but does not use information from the customer. This method is thus on the wrong object, it should be moved to the rental. To move a method you first copy the code over to rental, adjust it to fit in its new home and compile.