Oracle Exam 1Z0-808 Topic 18 Question 112 Discussion

Given these two classes:

Any amount of electricity used by a customer (represented by an instance of the Customer class) must contribute to the customer's bill (represented by the member variable bill) through the useElectricity method.

An instance of the Customer class should never be able to tamper with or decrease the value of the member variable bill.

How should you write methods in the ElectricAccount class at line n1 so that the member variable bill is always equal to the value of the member variable kwh multiplied by the member variable rate?