move amount calculation out of class customer and 
into class rental

the new method uses information from rental, and not from customer

do this in 2 steps... the first is here...

make the new method in the proper class.
go to the existing method... remove its body and
replace it with a delegation to the new method in
the other class.


compile, test, all is well.

This first step used the existing JUnit tests to verify
the new method works correctly.